Shulou(Shulou.com)06/03 Report--

This article mainly shows you "what are the JavaScript interview questions". The content is simple and clear. I hope it can help you solve your doubts. Let the editor lead you to study and learn this article "what are the JavaScript interview questions?"

The difference between undefined and not defined in question 1:JavaScript

Direct use of JavaScript undeclared variables throws an exception: var name is not defined, and if the exception is not handled, the code stops running. However, using typeof undeclared_variable does not generate an exception and returns undefined directly.

Var x; / / declare xconsole.log (x); / / output: undefined console.log (typeof y); / / output: undefined console.log (z); / / throw exception: ReferenceError: z is not defined

Question2: what does the following code output?

Var y = 1 if (function f () {}) {y + = typeof f;} console.log (y)

The correct answer should be 1undefined.

The evaluation of the if statement in JavaScript actually uses the eval function, and eval (function f () {}) returns function f () {}, which is true.

Next we can transform the code into its equivalent code.

Var k = 1 * * if (1) {eval (function foo () {}); k + = typeof foo;} console.log (k)

The code output above is actually 1undefined. Why is that? Let's look at the eval () documentation to get the answer.

This method only accepts the original string as a parameter, and if the string parameter is not the original string, then the method will return without any change.

It just so happens that the return value of the function f () {} statement is undefined, so everything makes sense.

Note that the above code is different from the following code.

Var k = 1 * * if (1) {function foo () {}; k + = typeof foo;} console.log (k); / / output 1function

Question 3: what are the disadvantages of creating a real private method in JavaScript?

Each object creates a method of the private method, which is memory-consuming

Observe the following code

Var Employee = function (name, company, salary) {this.name = name | | "; this.company = company | |"; this.salary = salary | | 5000; / Private method var increaseSalary = function () {this.salary = this.salary + 1000;}; / / Public method this.dispalyIncreasedSalary = function () {increaseSlary (); console.log (this.salary);};}; / / Create Employee class objectvar emp1 = new Employee ("John", "Pluto", 3000) / / Create Employee class objectvar emp2 = new Employee ("Merry", "Pluto", 2000); / / Create Employee class objectvar emp3 = new Employee ("Ren", "Pluto", 2500)

Here emp1,emp2,emp3 has a copy of the increaseSalary private method.

So we highly discourage using proprietary methods unless necessary.

What is a closure in question 4:JavaScript? Write an example

The old problem is that a closure declares another function in one function, and that function accesses variables in the scope of the parent function.

Here is an example of a closure that accesses variables in three fields

A variable with its own scope

Variables within the scope of the parent function

Variables with global scope

Var globalVar = "abc"; / / Parent self invoking function (function outerFunction (outerArg) {/ / begin of scope outerFunction / / Variable declared in outerFunction function scope var outerFuncVar = 'xcow; / / Closure self-invoking function (function innerFunction (innerArg) {/ / begin of scope innerFunction / / variable declared in innerFunction function scope var innerFuncVar = "y") Console.log ("outerArg =" + outerArg + "n" + "outerFuncVar =" + outerFuncVar + "n" + "innerArg =" + innerArg + "n" + "innerFuncVar =" + innerFuncVar + "n" + "globalVar =" + globalVar);} / / end of scope innerFunction) (5); / / Pass 5 as parameter} / / end of scope outerFunction) (7); / / Pass 7 as parameter innerFunction is closure that is defined inside outerFunc

The output is simple:

OuterArg = 7

OuterFuncVar = x

InnerArg = 5

InnerFuncVar = y

GlobalVar = abc

Question 5: write a mul function and use it as follows.

Console.log (mul (2) (3) (4)); / output: 24 console.log (mul (4) (3) (4)); / / output: 48

The answer is directly given:

Function mul (x) {return function (y) {/ / anonymous function return function (z) {/ / anonymous function return x * y * z;};}

To put it simply: mul returns an anonymous function, and running this anonymous function returns an anonymous function. The innermost anonymous function can access xQuery y Magnez and then calculate the product return.

For functions in JavaScript, you can generally consider the following knowledge points:

The function is a first-class citizen

A function can have properties and can be connected to its constructor

A function can be stored in memory like a variable

Functions can be passed to other functions as arguments

Function can return other functions

Question how does 6:JavaScript empty the array?

Such as

Var arrayList = ['axiajiajiaoyuanzhongyuanzhongyuanzhongyuanzhongyuanzhongyuanzhongyuanzhongyuanzhongyuanzhongyuanzhongyuanzhongyuanzhongyuanzhongyuanzhongyuanzhongyuanzhongyuanzhongyuanzhongyuanzhangzhuangzhuangzhuangzhuangzhuangzhuangzhuangzhuangzhongzhuangzhuangzhuangzhuangzhuangzhuangyuanzhongyuanzhongyuanzhangzhongyuanzhongyuanzhongyuanshangzhongyuanzhongyuanfangzhongyangzhong

How to empty arrayList

Method 1

ArrayList = []

Directly change the object that arrayList points to, and the original object does not change.

Method 2

ArrayList.length = 0

This method clears the elements of the original array by setting length=0.

Method 3

ArrayList.splice (0, arrayList.length)

Similar to method 2

Question 7: how to determine whether an object is an array?

Method 1

Use Object.prototype.toString to determine whether it is an array

Function isArray (obj) {return Object.prototype.toString.call (obj) = ='[object Array]';}

Call is used here to point the this in toString to obj. And then complete the judgment.

Method two

Use the prototype chain to complete the judgment

Function isArray (obj) {return obj.__proto__ = Array.prototype;}

The basic idea is to use an instance whose _ _ proto__ is the prototype attribute pointing to the constructor if it is constructed by a constructor.

Method 3

Using JQuery

Function isArray (obj) {return $.isArray (obj)}

The implementation of JQuery isArray is actually method 1.

Question 8: what does the following code output?

Var output = (function (x) {delete x; return x;}) (0); console.log (output)

The output is 0. The delete operator is an operation that deletes the attribute of object. But the x here is not an attribute of the object, and the delete operator does not work.

Question 9: what does the following code output?

Var x = 1bot var output = (function () {delete x; return x;}) (); console.log (output)

The output is 1. The delete operator is an operation that deletes the attribute of object. But the x here is not an attribute of the object, and the delete operator does not work.

Question 10: what does the following code output?

Var x = {foo: 1}; var output = (function () {delete x.foo; return x.foo;}) (); console.log (output)

The output is undefined. Although x is a global variable, it is an object. Delete acts on x.foo and successfully deletes x.foo. So return undefined

Question 11: what does the following code output?

Var Employee = {company: 'xyz'} var emp1 = Object.create (Employee); delete emp1.companyconsole.log (emp1.company)

The output is xyz, where emp1 inherits Employee's company through prototype. Emp1 itself does not have a company attribute. So the role of the delete operator is invalid.

Question 12: what is undefined x 1?

Execute the following code under chrome, and we can see the shadow of undefined x 1.

Var trees = ["redwood", "bay", "cedar", "oak", "maple"]; delete trees [3]; console.log (trees)

When we use the delete operator to delete an element from an array, the position of the element becomes a placeholder. Undefined x 1 is printed out. Note that if we use trees [3] = = 'undefined × 1', it returns false. Because it's just a printed representation, it doesn't change the value to undefined x 1.

Question 13: what does the following code output?

Var trees = ["xyz", "xxxx", "test", "ryan", "apple"]; delete trees [3]; console.log (trees.length)

The output is 5. Because the delete operator does not affect the length of the array.

Question 14: what does the following code output?

Var bar = true;console.log (bar + 0); console.log (bar + "xyz"); console.log (bar + true); console.log (bar + false)

The output is

one

Truexyz

two

one

Here is a table of addition operations

Number + Number-> addition

Boolean + Number-> addition

Boolean + Boolean-> addition

Number + String-> connection

String + Boolean-> connection

String + String-> connection

Question 15: what does the following code output?

Var z = 1, y = z = typeof ybot console.log (y)

The output is undefined. The association law of assignment operation in js is from right to left, that is, starting from the far right, the value is assigned to the variable on the left.

The above code is equivalent to

Var z = 1z = typeof y _ var y = z _ share console.log (y)

Question 16: what does the following code output?

Var foo = function bar () {return 12;}; typeof bar ()

The output is to throw an exception, bar is not defined. If you want the code to work properly, you need to modify the code like this:

Var bar = function () {return 12;}; typeof bar ()

Or

Function bar () {return 12;}; typeof bar ()

State clearly the next problem.

Var foo = function bar () {/ / foo is visible here// bar is visible here console.log (typeof bar ()); / / Work here:)}; / / foo is visible here// bar is undefined here

Question 17: what's the difference between the two function declarations?

Var foo = function () {/ / Some code}; function bar () {/ / Some code}

Foo is defined at run time. In order to explain this problem systematically, we should introduce the concept of variable promotion.

We can run the following code to see the result.

Console.log (foo) console.log (bar) var foo = function () {/ / Some code}; function bar () {/ / Some code}

Output as

Undefinedfunction bar () {/ / Some code}

Why is that? Why is it that foo prints out as undefined and bar prints as functions?

When JavaScript executes, it promotes the variable.

So the above code JavaScript engine executes in this order when it is actually executed.

/ / the defined position of foo bar is promoted to function bar () {/ / Some code}; var foo;console.log (foo) console.log (bar) foo = function () {/ / Some code}

The output of the original code is reasonably explained.

Question 18: what does the following code output?

Var salary = "1000 $"; (function () {console.log ("Original salary was" + salary); var salary = "5000 $"; console.log ("My New Salary" + salary);}) ()

The output is

Original salary was undefined

My New Salary 5000 $

This question also examines variable improvement. Is equivalent to the following code

Var salary = "1000 $"; (function () {var salary; console.log ("Original salary was" + salary); salary = "5000 $"; console.log ("My New Salary" + salary);}) ()

Question 19: what is the instanceof operator? What does the following code output?

Function foo () {return foo;} console.log (new foo () instanceof foo)

The instanceof operator is used to determine whether the current object is an object of a particular class.

Such as

Function Animal () {/ / or do not write return statement return this;} var dog = new Animal (); dog instanceof Animal / / Output: true

However, the foo here is defined as

Function foo () {return foo;}

So

/ / here bar is pointer to function foo () {return foo} .var bar = new foo ()

So new foo () instanceof foo returns false

Question 20: if we use JavaScript's associative array, how do we calculate the length of the associative array?

Var counterArray = {A: 3, B: 4}; counterArray ["C"] = 1

In fact, the answer is very simple, just calculate the number of key directly.

Object.keys (counterArray). Length / / Output 3 and above are all the contents of the article "what are the JavaScript interview questions?" Thank you for reading! I believe we all have a certain understanding, hope to share the content to help you, if you want to learn more knowledge, welcome to follow the industry information channel!

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.