100 Advanced JavaScript Multiple Choice Questions with Answers and Explanations
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JavaScript is a versatile and powerful language used widely for web development. To truly master JavaScript, especially its advanced concepts, it's essential to understand not just the syntax but the underlying principles and behaviors of the language. Below are 100 advanced multiple-choice questions designed to challenge your knowledge and help you gain a deeper understanding of JavaScript. Each question is accompanied by detailed explanations of the correct answers.
Questions and Answers
Question: What will the following code output?
console.log(1 + "1" - 1);
a) "10"
b) 10
c) 0
d) "11"
Answer: c) 0
Explanation: In JavaScript, the + operator with a string and a number results in concatenation, making 1 + "1" equal to "11". When "11" is subtracted by 1, JavaScript implicitly converts "11" to a number, resulting in 11 - 1, which is 10.
Question: What will be the output of the following code?
(function() {
var a = b = 5;
})();
console.log(b);
a) ReferenceError
b) 5
c) undefined
d) TypeError
Answer: b) 5
Explanation: The expression var a = b = 5 is evaluated as b = 5 followed by var a = b. Here, b becomes a global variable because it is not declared with var. Hence, console.log(b) outputs 5.
Question: What is the result of the following code?
console.log([] + []);
a) ""
b) "[]"
c) "[object Object][object Object]"
d) NaN
Answer: a) ""
Explanation: In JavaScript, the + operator between two arrays results in their string representation being concatenated. The string representation of an empty array is an empty string, so [] + [] results in "".
Question: What will the following code output?
console.log(typeof NaN);
a) "number"
b) "NaN"
c) "undefined"
d) "object"
Answer: a) "number"
Explanation: NaN stands for "Not-a-Number", but its type in JavaScript is actually number.
Question: What will be the output of the following code?
console.log(0.1 + 0.2 === 0.3);
a) true
b) false
Answer: b) false
Explanation: Due to floating-point precision issues in JavaScript, 0.1 + 0.2 does not exactly equal 0.3, but rather a number very close to it, hence the comparison returns false.
Question: What will the following code output?
var foo = {n: 1};
var bar = foo;
foo.x = foo = {n: 2};
console.log(foo.x);
a) undefined
b) {n: 2}
c) {n: 1}
d) TypeError
Answer: a) undefined
Explanation: The assignment foo.x = foo = {n: 2} is evaluated from right to left. First, foo is assigned {n: 2}. However, the previous foo (which is referenced by bar) has its x property set to the new foo, which is {n: 2}. Since the new foo does not have an x property, foo.x is undefined.
Question: What is the output of the following code?
console.log(typeof typeof 1);
a) "number"
b) "string"
c) "object"
d) "undefined"
Answer: b) "string"
Explanation: The typeof operator always returns a string. So typeof 1 returns "number", and typeof "number" returns "string".
Question: What will the following code output?
function foo() {
return;
{
test: 1
};
}
console.log(typeof foo());
a) "undefined"
b) "object"
c) "number"
d) "function"
Answer: a) "undefined"
Explanation: Due to automatic semicolon insertion, return; is treated as a complete statement. Therefore, the function returns undefined.
Question: What will be the output of the following code?
var a = 1;
function b() {
a = 10;
return;
function a() {}
}
b();
console.log(a);
a) 1
b) 10
c) undefined
d) ReferenceError
Answer: a) 1
Explanation: In the function b, the declaration function a() {} is hoisted to the top of the function scope, so a inside b is a local variable. The assignment a = 10 updates this local a, not the global a. Therefore, the global a remains 1.
Question: What will the following code output?
var a = 5;
(function() {
var a = 6;
console.log(a);
a = 10;
console.log(a);
})();
console.log(a);
a) 5, 6, 10
b) 6, 10, 5
c) 6, 10, 10
d) 5, 6, 10, 5
Answer: b) 6, 10, 5
Explanation: The first console.log(a) inside the function outputs 6 because a is locally declared. After assigning 10 to the local a, the second console.log(a) outputs 10. The console.log(a) outside the function outputs the global a, which remains 5.
Question: What will the following code output?
var a = 2;
var b = (function() {
delete a;
return a;
})();
console.log(a);
console.log(b);
a) 2, undefined
b) undefined, undefined
c) 2, 2
d) undefined, 2
Answer: c) 2, 2
Explanation: The delete operator is used to remove properties from objects. Here, a is a variable, not a property of an object, so delete a has no effect. Thus, a remains 2, and the returned value b is also 2.
Question: What will the following code output?
var x = [1, 2, 3];
x[10] = 99;
console.log(x.length);
a) 3
b) 10
c) 11
d) 4
Answer: c) 11
Explanation: Setting a value at index 10 increases the length of the array to 11, because JavaScript arrays are sparse, and unassigned indexes between the highest index and the array's length are treated as undefined.
Question: What will the following code output?
var arr = [10, 20, 30];
arr['key'] = 'value';
console.log(arr.length);
a) 3
b) 4
c) 1
d) undefined
Answer: a) 3
Explanation: Setting a property arr['key'] does not affect the array's length property, which only counts numeric indices. Therefore, the length remains 3.
Question: What will the following code output?
function test() {
console.log(a);
console.log(foo());
var a = 1;
function foo() {
return 2;
}
}
test();
a) 1, 2
b) undefined, 2
c) ReferenceError
d) undefined, ReferenceError
Answer: b) undefined, 2
Explanation: Variables declared with var are hoisted to the top of the function scope but are not initialized. Thus, a is undefined when console.log(a) is called. Function declarations are also hoisted, so foo is available and returns 2.
Question: What will be the output of the following code?
function f() {}
console.log(f.prototype);
a) undefined
b) {}
c) null
d) [object Object]
Answer: b) {}
Explanation: Every function in JavaScript has a prototype property, which is an empty object by default.
Question: What will the following code output?
var F = function() {};
var f = new F();
console.log(F.prototype === f.__proto__);
a) true
b) false
Answer: a) true
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var a = {};
var b = {key: 'b'};
var c = {key: 'c'};
a[b] = 123;
a[c] = 456;
console.log(a[b]);
- a) 123
- b) 456
- c) undefined
- d) TypeError
Answer: b) 456
Explanation: When objects are used as keys in JavaScript, they are converted to strings (`[object Object]`). Therefore, both a[b] and a[c] refer to the same key, and the last assignment overwrites the previous one. Thus, a[b] outputs 456.
Question: What will be the output of the following code?
var x = 10;
(function() {
console.log(x);
var x = 20;
})();
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: c) undefined
Explanation: The variable x is hoisted within the function scope, so the console.log(x) references the hoisted but uninitialized x, which is undefined.
Question: What will the following code output?
function foo() {
return
{
message: "Hello"
};
}
console.log(foo());
- a) { message: "Hello" }
- b) undefined
- c) null
- d) SyntaxError
Answer: b) undefined
Explanation: Due to automatic semicolon insertion, the return statement is terminated before the object is returned. Hence, foo() returns undefined.
Question: What will the following code output?
var f = function g() {
return 23;
};
console.log(typeof g());
- a) "number"
- b) "undefined"
- c) "function"
- d) ReferenceError
Answer: d) ReferenceError
Explanation: The function g is only accessible within its own scope. Outside of this scope, it is not defined, resulting in a ReferenceError.
Question: What will the following code output?
var length = 10;
function fn() {
console.log(this.length);
}
var obj = {
length: 5,
method: function() {
fn();
arguments[0]();
}
};
obj.method(fn);
- a) 10, 5
- b) 5, 10
- c) 10, 10
- d) 5, 5
Answer: c) 10, 10
Explanation: When fn is called in obj.method(fn), the this keyword inside fn refers to the global object, where length is 10. The arguments[0]() call also sets this to the global object, resulting in 10.
Question: What will the following code output?
function test() {
var a = b = 3;
}
test();
console.log(typeof a);
console.log(typeof b);
- a) "undefined", "number"
- b) "number", "number"
- c) "undefined", "undefined"
- d) "number", "undefined"
Answer: a) "undefined", "number"
Explanation: The statement var a = b = 3 declares a locally within the function, but b is implicitly declared as a global variable. Thus, a is undefined outside the function, while b is number.
Question: What will the following code output?
var foo = function bar() { return 42; };
console.log(typeof bar);
- a) "number"
- b) "function"
- c) "undefined"
- d) "object"
Answer: c) "undefined"
Explanation: The function bar is only available within its own scope. Outside this scope, it is not defined, resulting in typeof bar being undefined.
Question: What will be the output of the following code?
function foo() {
var x = 10;
(function() {
console.log(x);
})();
x = 20;
}
foo();
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: a) 10
Explanation: The immediately invoked function expression (IIFE) captures the initial value of x, which is 10, before it is reassigned to 20.
Question: What will the following code output?
var a = 1;
var obj = {
a: 2,
fn: function() {
console.log(this.a);
}
};
var fn = obj.fn;
fn();
- a) 1
- b) 2
- c) undefined
- d) ReferenceError
Answer: a) 1
Explanation: When fn is called outside the context of obj, this refers to the global object, where a is 1.
Question: What will the following code output?
var obj = {
a: 2,
foo: function() {
var self = this;
(function() {
console.log(self.a);
})();
}
};
obj.foo();
- a) undefined
- b) 2
- c) ReferenceError
- d) null
Answer: b) 2
Explanation: The variable self captures the this context of obj, allowing the inner function to correctly reference obj.a.
Question: What will the following code output?
var obj = {
a: 1,
foo: function() {
console.log(this.a);
}
};
setTimeout(obj.foo, 1000);
- a) 1
- b) undefined
- c) null
- d) 0
Answer: b) undefined
Explanation: When obj.foo is passed to setTimeout, the context (`this`) is lost, so this.a refers to undefined.
Question: What will the following code output?
var obj = {
a: 1,
foo: function() {
console.log(this.a);
}
};
setTimeout(function() {
obj.foo();
}, 1000);
- a) 1
- b) undefined
- c) null
- d) 0
Answer: a) 1
Explanation: In this case, obj.foo is called within a function where obj is correctly referenced, so this.a is 1.
Question: What will the following code output?
function foo() {
console.log(this);
}
foo.call(null);
- a) null
- b) undefined
- c) global object (window in browsers)
- d) TypeError
Answer: c) global object (window in browsers)
Explanation: When null is passed as the context to call, this defaults to the global object.
Question: What will the following code output?
function foo() {
"use strict";
console.log(this);
}
foo.call(null);
- a) null
- b) undefined
- c) global object (window in browsers)
- d) TypeError
Answer: a) null
Explanation: In strict mode, passing null to call or apply results in this being null.
Question: What will the following code output?
var obj = {
a: 1,
foo: function() {
return function() {
console.log(this.a);
};
}
};
var fn = obj.foo();
fn();
- a) 1
- b) undefined
- c) null
- d) 0
Answer: b) undefined
Explanation: The inner function loses the this context of obj and refers to the global object, where a is undefined.
Question: What will the following code output?
var obj = {
a: 1,
foo: function() {
return () => {
console.log(this.a);
};
}
};
var fn = obj.foo();
fn();
- a) 1
- b) undefined
- c) null
- d) 0
Answer: a) 1
Explanation: Arrow functions capture the this context of their surrounding scope, which in this case is obj.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
console.log(this.a);
setTimeout(function() {
console.log(this.a);
}, 1000);
}
};
obj.foo();
- a) 20, 10
- b) 20, 20
- c) undefined, 10
- d) undefined, undefined
Answer: a) 20, 10
Explanation: The first console.log(this.a) refers to obj.a, which is 20. The setTimeout callback has this referring to the global object, where a is 10.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
var self = this;
console.log(this.a);
setTimeout(function() {
console.log(self.a);
}, 1000);
}
};
obj.foo();
- a) 20, 10
- b) 20, 20
- c) undefined, 10
- d) undefined, undefined
Answer: b) 20, 20
Explanation: The variable self captures the this context of obj, allowing the setTimeout callback to correctly reference obj.a.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
console.log(this.a);
setTimeout(() => {
console.log(this.a);
}, 1000);
}
};
obj.foo();
- a) 20, 10
- b) 20, 20
- c) undefined, 10
- d) undefined, undefined
Answer: b) 20, 20
Explanation: Arrow functions capture the this context of their surrounding scope, which in this case is obj.
Question: What will the following code output?
function Foo() {}
Foo.prototype.bar = 10;
var foo = new Foo();
Foo.prototype.bar = 20;
console.log(foo.bar);
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: b) 20
Explanation: Changing the prototype property of Foo after an instance has been created affects the instance, so foo.bar is 20.
Question: What will the following code output?
function Foo() {
this.bar = 10;
}
Foo.prototype.bar = 20;
var foo = new Foo();
console.log(foo.bar);
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: a) 10
Explanation: The instance property bar of foo shadows the prototype property, so foo.bar is 10.
Question: What will the following code output?
function Foo() {}
Foo.prototype = {bar: 10};
var foo = new Foo();
Foo.prototype.bar = 20;
console.log(foo.bar);
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: b) 20
Explanation: Changing the property of the prototype object affects the instance, so foo.bar is 20.
Question: What will the following code output?
function Foo() {}
Foo.prototype = {bar: 10};
var foo = new Foo();
Foo.prototype = {bar: 20};
console.log(foo.bar);
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: a) 10
Explanation: Reassigning the prototype object does not affect existing instances, so foo.bar remains 10.
Question: What will the following code output?
function Foo() {}
Foo.prototype = {bar: 10};
var foo1 = new Foo();
Foo.prototype = {bar: 20};
var foo2 = new Foo();
- a) 10, 10
- b) 20, 20
- c) 10, 20
- d) undefined, undefined
Answer: c) 10, 20
Explanation: foo1 is created with the initial prototype, so foo1.bar is 10. foo2 is created after the prototype reassignment, so foo2.bar is 20.
Question: What will the following code output?
function Foo() {}
Foo.prototype.bar = 10;
var foo1 = new Foo();
Foo.prototype = {bar: 20};
var foo2 = new Foo();
- a) 10, 10
- b) 20, 20
- c) 10, 20
- d) undefined, undefined
Answer: c) 10, 20
Explanation: foo1 retains the initial prototype, so foo1.bar is 10. foo2 is created with the reassigned prototype, so foo2.bar is 20.
Question: What will the following code output?
function Foo() {}
Foo.prototype.bar = 10;
var foo = new Foo();
Foo.prototype.baz = 20;
console.log(foo.baz);
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: b) 20
Explanation: Adding properties to the prototype after an instance has been created affects the instance, so foo.baz is 20.
Question: What will the following code output?
function Foo() {
this.bar = 10;
}
Foo.prototype.baz = 20;
var foo = new Foo();
delete foo.baz;
console.log(foo.baz);
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: b) 20
Explanation: Deleting a non-existent property has no effect, so foo.baz still refers to the prototype property, which is 20.
Question: What will the following code output?
function Foo() {
this.bar = 10;
}
Foo.prototype.baz = 20;
var foo = new Foo();
delete foo.bar;
console.log(foo.bar);
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: c) undefined
Explanation: Deleting an instance property removes it from the instance, so foo.bar is undefined.
Question: What will the following code output?
function Foo() {
this.bar = 10;
}
Foo.prototype.baz = 20;
var foo = new Foo();
delete foo.bar;
foo.bar = 30;
console.log(foo.bar);
- a) 10
- b) 20
- c) 30
- d) undefined
Answer: c) 30
Explanation: Reassigning the bar property on the instance sets it to 30.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
var a = 30;
return this.a;
}
};
console.log(obj.foo());
- a) 10
- b) 20
- c) 30
- d) undefined
Answer: b) 20
Explanation: The this context within foo refers to obj, so this.a is 20.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
var a = 30;
return function() {
return this.a;
};
}
};
console.log(obj.foo()());
- a) 10
- b) 20
- c) 30
- d) undefined
Answer: a) 10
Explanation: The returned function has its this context set to the global object, so this.a is 10.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
var a = 30;
return () => {
return this.a;
};
}
};
console.log(obj.foo()());
- a) 10
- b) 20
- c) 30
- d) undefined
Answer: b) 20
Explanation: Arrow functions capture the this context of their surrounding scope, which in this case is obj.
Question: What will the following code output?
var obj = {
foo: function() {
return () => {
return this;
};
}
};
console.log(obj.foo()() === obj);
- a) true
- b) false
Answer: a) true
Explanation: The arrow function captures the this context of obj, so it returns obj.
Question: What will the following code output?
var obj = {
foo: function() {
return function() {
return this;
};
}
};
console.log(obj.foo()() === obj);
- a) true
- b) false
Answer: b) false
Explanation: The returned function has its this context set to the global object, so it does not return obj.
Question: What will the following code output?
var obj = {
foo: function() {
return this;
}
};
var bar = obj.foo;
console.log(bar() === obj);
- a) true
- b) false
Answer: b) false
Explanation: When bar is called, this refers to the global object, not obj.
Question: What will the following code output?
var obj = {
foo: function() {
return this;
}
};
console.log(obj.foo() === obj);
- a) true
- b) false
Answer: a) true
Explanation: When obj.foo is called, this refers to obj, so it returns obj.
Question: What will the following code output?
var obj = {
foo: function() {
return this;
}
};
var bar = obj.foo;
console.log(bar.call(obj) === obj);
- a) true
- b) false
Answer: a) true
Explanation: When bar is called with call(obj), this is explicitly set to obj, so it returns obj.
Question: What will the following code output?
var obj = {
foo: function() {
return this;
}
};
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var bar = obj.foo;
console.log(bar.apply(obj) === obj);
- a) true
- b) false
Answer: a) true
Explanation: When bar is called with apply(obj), this is explicitly set to obj, so it returns obj.
Question: What will the following code output?
var obj = {
foo: function() {
return this;
}
};
var bar = obj.foo;
console.log(bar.bind(obj)() === obj);
- a) true
- b) false
Answer: a) true
Explanation: When bar is called with bind(obj), this is permanently bound to obj, so it returns obj.
Question: What will the following code output?
var obj = {
foo: function() {
return () => {
return this;
};
}
};
var bar = obj.foo;
console.log(bar()() === obj);
- a) true
- b) false
Answer: b) false
Explanation: The arrow function captures the this context of bar, which is the global object, not obj.
Question: What will the following code output?
var obj = {
foo: function() {
return () => {
return this;
};
}
};
console.log(obj.foo()() === obj);
- a) true
- b) false
Answer: a) true
Explanation: The arrow function captures the this context of obj, so it returns obj.
Question: What will the following code output?
var obj = {
foo: function() {
return () => {
return this.a;
};
},
a: 10
};
console.log(obj.foo()());
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: a) 10
Explanation: The arrow function captures the this context of obj, so it correctly references obj.a, which is 10.
Question: What will the following code output?
var obj = {
foo: function() {
return function() {
return this.a;
};
},
a: 10
};
console.log(obj.foo()());
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: b) undefined
Explanation: The returned function has its this context set to the global object, where a is undefined.
Question: What will the following code output?
var obj = {
foo: function() {
return function() {
return this.a;
};
},
a: 10
};
console.log(obj.foo.call({a: 20})());
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: b) 20
Explanation: The call method sets the this context for foo, so this.a is 20.
Question: What will the following code output?
var obj = {
foo: function() {
return function() {
return this.a;
};
},
a: 10
};
console.log(obj.foo.apply({a: 30})());
- a) 10
- b) 30
- c) undefined
- d) ReferenceError
Answer: b) 30
Explanation: The apply method sets the this context for foo, so this.a is 30.
Question: What will the following code output?
var obj = {
foo: function() {
return function() {
return this.a;
};
},
a: 10
};
console.log(obj.foo.bind({a: 40})());
- a) 10
- b) 40
- c) undefined
- d) ReferenceError
Answer: b) 40
Explanation: The bind method sets the this context for foo permanently, so this.a is 40.
Question: What will the following code output?
var obj = {
foo: function() {
return () => {
return this.a;
};
},
a: 10
};
console.log(obj.foo.call({a: 50})());
- a) 10
- b) 50
- c) undefined
- d) ReferenceError
Answer: a) 10
Explanation: The arrow function captures the this context of obj, so this.a is 10.
Question: What will the following code output?
var obj = {
foo: function() {
return () => {
return this.a;
};
},
a: 10
};
console.log(obj.foo.apply({a: 60})());
- a) 10
- b) 60
- c) undefined
- d) ReferenceError
Answer: a) 10
Explanation: The arrow function captures the this context of obj, so this.a is 10.
Question: What will the following code output?
var obj = {
foo: function() {
return () => {
return this.a;
};
},
a: 10
};
console.log(obj.foo.bind({a: 70})());
- a) 10
- b) 70
- c) undefined
- d) ReferenceError
Answer: a) 10
Explanation: The arrow function captures the this context of obj, so this.a is 10.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
console.log(this.a);
}
};
var bar = obj.foo;
bar.call(obj);
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: b) 20
Explanation: The call method sets the this context to obj, so this.a is 20.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
console.log(this.a);
}
};
var bar = obj.foo;
bar.apply(obj);
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: b) 20
Explanation: The apply method sets the this context to obj, so this.a is 20.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
console.log(this.a);
}
};
var bar = obj.foo;
bar.bind(obj)();
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: b) 20
Explanation: The bind method sets the this context to obj, so this.a is 20.
Question: What will the following code output?
var obj = {
foo: function() {
return () => {
return this.a;
};
},
a: 10
};
var bar = obj.foo;
console.log(bar()());
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: b) undefined
Explanation: The arrow function captures the this context of the global object, where a is undefined.
Question: What will the following code output?
var obj = {
foo: function() {
return () => {
return this.a;
};
},
a: 10
};
console.log(obj.foo()());
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: a) 10
Explanation: The arrow function captures the this context of obj, so this.a is 10.
Question: What will the following code output?
function Foo() {
this.bar = 10;
}
Foo.prototype.baz = function() {
return this.bar;
};
var foo = new Foo();
console.log(foo.baz());
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: a) 10
Explanation: The method baz on the prototype correctly references the instance property bar, so foo.baz() returns 10.
Question: What will the following code output?
function Foo() {
this.bar = 10;
}
Foo.prototype.baz = function() {
return this.bar;
};
var foo = new Foo();
var baz = foo.baz;
console.log(baz());
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: b) undefined
Explanation: The this context of baz is the global object, where bar is undefined.
Question: What will the following code output?
function Foo() {
this.bar = 10;
}
Foo.prototype.baz = function() {
return this.bar;
};
var foo = new Foo();
var baz = foo.baz.bind(foo);
console.log(baz());
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: a) 10
Explanation: The bind method sets the this context of baz to foo, so it correctly references foo.bar.
Question: What will the following code output?
function Foo() {
this.bar = 10;
}
Foo.prototype.baz = function() {
return this.bar;
};
var foo = new Foo();
setTimeout(foo.baz, 1000);
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: b) undefined
Explanation: The this context of baz is the global object when called inside setTimeout, where bar is undefined.
Question: What will the following code output?
function Foo() {
this.bar = 10;
}
Foo.prototype.baz = function() {
return this.bar;
};
var foo = new Foo();
setTimeout(foo.baz.bind(foo), 1000);
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: a) 10
Explanation: The bind method sets the this context of baz to foo, so it correctly references foo.bar.
Question: What will the following code output?
function Foo() {}
Foo.prototype.bar = 10;
var foo1 = new Foo();
Foo.prototype = {baz: 20};
var foo2 = new Foo();
- a) 10, undefined
- b) undefined, undefined
- c) 10, 10
- d) 10, 20
Answer: a) 10, undefined
Explanation: foo1 retains the initial prototype, so foo1.bar is 10. foo2 is created with the reassigned prototype, where bar is undefined.
Question: What will the following code output?
function Foo() {}
Foo.prototype.bar = 10;
var foo1 = new Foo();
Foo.prototype.bar = 20;
var foo2 = new Foo();
- a) 10, 10
- b) 20, 20
- c) 10, 20
- d) undefined, undefined
Answer: b) 20, 20
Explanation: Changing the prototype property affects all instances, so both foo1.bar and foo2.bar are 20.
Question: What will the following code output?
function Foo() {}
Foo.prototype.bar = 10;
var foo1 = new Foo();
Foo.prototype = {bar: 20};
var foo2 = new Foo();
- a) 10, 10
- b) 20, 20
- c) 10, 20
- d) undefined, undefined
Answer: c) 10, 20
Explanation: foo1 retains the initial prototype, so foo1.bar is 10. foo2 is created with the reassigned prototype, so foo2.bar is 20.
Question: What will the following code output?
function Foo() {}
Foo.prototype.bar = 10;
var foo1 = new Foo();
Foo.prototype = {baz: 20};
var foo2 = new Foo();
console.log(foo1.baz, foo2.baz);
- a) 20, 20
- b) undefined, 20
- c) undefined, undefined
- d) ReferenceError
Answer: b) undefined, 20
Explanation: foo1 retains the initial prototype, so foo1.baz is undefined. foo2 is created with the reassigned prototype, so foo2.baz is 20.
Question: What will the following code output?
function Foo() {}
Foo.prototype.bar = 10;
var foo1 = new Foo();
Foo.prototype.bar = 20;
var foo2 = new Foo();
- a) 10, 10
- b) 20, 20
- c) 10, 20
- d) undefined, undefined
Answer: b) 20, 20
Explanation: Changing the prototype property affects all instances, so both foo1.bar and foo2.bar are 20.
Question: What will the following code output?
function Foo() {}
Foo.prototype.bar = 10;
var foo1 = new Foo();
foo1.bar = 30;
Foo.prototype.bar = 20;
var foo2 = new Foo();
- a) 30, 10
- b) 30, 20
- c) 10, 20
- d) undefined, undefined
Answer: b) 30, 20
Explanation: The instance property foo1.bar shadows the prototype property, so foo1.bar is 30. The prototype property foo2.bar is 20.
Question: What will the following code output?
function Foo() {}
Foo.prototype.bar = 10;
var foo1 = new Foo();
Foo.prototype = {baz: 20};
var foo2 = new Foo();
- a) 10, undefined
- b) undefined, undefined
- c) 10, 10
- d) 10, 20
Answer: a) 10, undefined
Explanation: foo1 retains the initial prototype, so foo1.bar is 10. foo2 is created with the reassigned prototype, where bar is undefined.
Question: What will the following code output?
function Foo() {}
Foo.prototype.bar = 10;
var foo1 = new Foo();
Foo.prototype.bar = 20;
var foo2 = new Foo();
- a) 10, 10
- b) 20, 20
- c) 10, 20
- d) undefined, undefined
Answer: b) 20, 20
Explanation: Changing the prototype property affects all instances, so both foo1.bar and foo2.bar are 20.
Question: What will the following code output?
function Foo() {}
Foo.prototype.bar = 10;
var foo1 = new Foo();
Foo.prototype = {baz: 20};
var foo2 = new Foo();
console.log(foo1.baz, foo2.baz);
- a) 20, 20
- b) undefined, 20
- c) undefined, undefined
- d) ReferenceError
Answer: b) undefined, 20
Explanation: foo1 retains the initial prototype, so foo1.baz is undefined. foo2 is created with the reassigned prototype, so foo2.baz is 20.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
console.log(this.a);
}
};
var bar = obj.foo;
bar();
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: a) 10
Explanation: The this context of bar is the global object, so this.a is 10.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
console.log(this.a);
}
};
var bar = obj.foo;
bar.call(obj);
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: b) 20
Explanation: The call method sets the this context to obj, so this.a is 20.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
console.log(this.a);
}
};
var bar = obj.foo;
bar.apply(obj);
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: b) 20
Explanation: The apply method sets the this context to obj, so this.a is 20.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
console.log(this.a);
}
};
var bar = obj.foo;
bar.bind(obj)();
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: b) 20
Explanation: The bind method sets the this context to obj, so this.a is 20.
Question: What will the following code output?
var obj = {
foo: function() {
return () => {
return this.a;
};
},
a: 10
};
var bar = obj.foo;
console.log(bar()());
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: b) undefined
Explanation: The arrow function captures the this context of the global object, where a is undefined.
Question: What will the following code output?
var obj = {
foo: function() {
return () => {
return this.a;
};
},
a: 10
};
console.log(obj.foo()());
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: a) 10
Explanation: The arrow function captures the this context of obj, so this.a is 10.
Question: What will the following code output?
function Foo() {
this.bar = 10;
}
Foo.prototype.baz = function() {
return this.bar;
};
var foo = new Foo();
console.log(foo.baz());
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: a) 10
Explanation: The method baz on the prototype correctly references the instance property bar, so foo.baz() returns 10.
Question: What will the following code output?
function Foo() {
this.bar = 10;
}
Foo.prototype.baz = function() {
return this.bar;
};
var foo = new Foo();
var baz = foo.baz;
console.log(baz());
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: b) undefined
Explanation: The this context of baz is the global object, where bar is undefined.
Question: What will the following code output?
function Foo() {
this.bar = 10;
}
Foo.prototype.baz = function() {
return this.bar;
};
var foo = new Foo();
var baz = foo.baz.bind(foo);
console.log(baz());
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: a) 10
Explanation: The bind method sets the this context of baz to foo, so it correctly references foo.bar.
Question: What will the following code output?
function Foo() {
this.bar = 10;
}
Foo.prototype.baz = function() {
return this.bar;
};
var foo = new Foo();
setTimeout(foo.baz, 1000);
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: b) undefined
Explanation: The this context of baz is the global object when called inside setTimeout, where bar is undefined.
Question: What will the following code output?
function Foo() {
this.bar = 10;
}
Foo.prototype.baz = function() {
return this.bar;
};
var foo = new Foo();
setTimeout(foo.baz.bind(foo), 1000);
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: a) 10
Explanation: The bind method sets the this context of baz to foo, so it correctly references foo.bar.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
console.log(this.a);
}
};
var bar = obj.foo;
bar();
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: a) 10
Explanation: The this context of bar is the global object, so this.a is 10.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
console.log(this.a);
}
};
var bar = obj.foo;
bar.call(obj);
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: b) 20
Explanation: The call method sets the this context to obj, so this.a is 20.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
console.log(this.a);
}
};
var bar = obj.foo;
bar.apply(obj);
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: b) 20
Explanation: The apply method sets the this context to obj, so this.a is 20.
Question: What will the following code output?
var a = 10;
var obj = {
a: 20,
foo: function() {
console.log(this.a);
}
};
var bar = obj.foo;
bar.bind(obj)();
- a) 10
- b) 20
- c) undefined
- d) ReferenceError
Answer: b) 20
Explanation: The bind method sets the this context to obj, so this.a is 20.
Question: What will the following code output?
var obj = {
foo: function() {
return () => {
return this.a;
};
},
a: 10
};
var bar = obj.foo;
console.log(bar()());
- a) 10
- b) undefined
- c) null
- d) ReferenceError
Answer: b) undefined
Explanation: The arrow function captures the this context of the global object, where a is undefined.
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