Seven Questions and Answers to JavaScript Interview Questions That Are Easily Overlooked_Javascript Skills

This question is the last question in a set of front-end interview questions I asked. It is used to test the interviewer's comprehensive JavaScript ability. Unfortunately, in the past two years so far, almost no one can answer it completely. It is not It's difficult just because most interviewers underestimate him.

The topic is as follows:

function Foo() {
 getName = function () { alert (1); };
 return this;
}
Foo.getName = function () { alert (2);};
Foo.prototype.getName = function () { alert (3);};
var getName = function () { alert (4);};
function getName() { alert (5);}

//请写出以下输出结果：
Foo.getName();
getName();
Foo().getName();
getName();
new Foo.getName();
new Foo().getName();
new new Foo().getName();

The answer is:

function Foo() {
 getName = function () { alert (1); };
 return this;
}
Foo.getName = function () { alert (2);};
Foo.prototype.getName = function () { alert (3);};
var getName = function () { alert (4);};
function getName() { alert (5);}

//答案：
Foo.getName();//2
getName();//4
Foo().getName();//1
getName();//1
new Foo.getName();//2
new Foo().getName();//3
new new Foo().getName();//3

This question is based on my previous development experience and various JS pitfalls I encountered. This question involves many knowledge points, including variable definition promotion, this pointer pointing, operator priority, prototype, inheritance, global variable pollution, object attribute and prototype attribute priority, etc.

This question contains 7 questions, please explain them below.

First question

Let’s first look at what was done in the first half of this question. First, we defined a function called Foo, then created a static property called getName for Foo to store an anonymous function, and then created a new prototype object for Foo. An anonymous function called getName. Then a getName function is created through the function variable expression, and finally a getName function is declared.

The first question, Foo.getName, naturally accesses the static properties stored on the Foo function, which is naturally 2. There is nothing to say.

Second question

The second question is to call the getName function directly. Since it is called directly, it is accessing the function called getName in the current scope above, so it has nothing to do with 1 2 3. Many interviewers answered this question as 5. There are two pitfalls here, one is variable declaration promotion, and the other is function expression.

1. Variable declaration improvement

That is, all declared variables or declared functions will be promoted to the top of the current function.

For example, the following code:

console.log('x' in window);//true
var x;
x = 0;

When the code is executed, the js engine will raise the declaration statement to the top of the code and become:

var x;
console.log('x' in window);//true
x = 0;

2. Function expression

var getName and function getName are both declaration statements. The difference is that var getName is a function expression, while function getName is a function declaration. For more information on how to create various functions in JS, you can read the classic JS closure interview questions that most people do wrong. This article has detailed explanations.

The biggest problem with function expressions is that js will split this code into two lines of code and execute them separately.

For example, the following code:

console.log(x);//输出：function x(){}
var x=1;
function x(){}

The actual executed code is to first split var x=1 into two lines: var x; and x = 1;, and then raise the two lines var x; and function x(){} to the top to become:

var x;
function x(){}
console.log(x);
x=1;

So the x declared by the final function covers the x declared by the variable, and the log output is the x function.

Similarly, the final execution of the code in the original question is:

function Foo() {
 getName = function () { alert (1); };
 return this;
}
var getName;//只提升变量声明
function getName() { alert (5);}//提升函数声明，覆盖var的声明

Foo.getName = function () { alert (2);};
Foo.prototype.getName = function () { alert (3);};
getName = function () { alert (4);};//最终的赋值再次覆盖function getName声明

getName();//最终输出4

Third question

The third question, Foo().getName(); first executes the Foo function, and then calls the getName attribute function of the return value object of the Foo function.

The first sentence of the Foo function getName = function () { alert (1); }; is a function assignment statement. Note that it does not have a var declaration, so first look for the getName variable in the current Foo function scope, and there is none. Then look to the upper layer of the current function scope, that is, the outer scope, to find whether it contains the getName variable. It is found, which is the alert(4) function in the second question. Assign the value of this variable to function(){alert(1) }.

Here is actually the getName function in the outer scope that is modified.

Note: If it is still not found here, it will search all the way up to the window object. If there is no getName attribute in the window object, create a getName variable in the window object.

After that, the return value of the Foo function is this, and there are already many articles on the this problem of JS in the blog garden, so I won’t go into more details here.

To put it simply, the point of this is determined by the calling method of the function. In the direct calling method here, this points to the window object.

The Foo function returns the window object, which is equivalent to executing window.getName(), and the getName in the window has been modified to alert(1), so 1 will be output in the end

Two knowledge points are examined here, one is the issue of variable scope and the other is the issue of this pointing.

Question 4

Call the getName function directly, which is equivalent to window.getName(), because this variable has been modified when the Foo function is executed, and the result is the same as the third question, which is 1

Fifth question

The fifth question is new Foo.getName(); , what is examined here is the operator priority issue of js.

By looking up the table above, we can know that the priority of point (.) is higher than the new operation, which is equivalent to:

new (Foo.getName)();
So the getName function is actually executed as a constructor, and 2 pops up.

Question 6

The sixth question is new Foo().getName(). First of all, the operator precedence brackets are higher than new. The actual execution is

(new Foo()).getName()
Then the Foo function is executed first, and Foo, as a constructor, has a return value, so here we need to explain the return value of the constructor in js.

Constructor return value

In traditional languages, constructors should not have a return value. The return value of the actual execution is the instantiated object of this constructor.

In js, constructors can have return values ​​or not.

1. If there is no return value, the instantiated object will be returned as in other languages.

2. If there is a return value, check whether the return value is a reference type. If it is a non-reference type, such as a basic type (string, number, boolean, null, undefined), it is the same as no return value, and its instantiated object is actually returned.

3. If the return value is a reference type, the actual return value is this reference type.

In the original question, this is returned, and this originally represents the current instantiated object in the constructor, so the Foo function finally returns the instantiated object.

Then call the getName function of the instantiated object. Because no attributes are added to the instantiated object in the Foo constructor, we look for getName in the prototype object of the current object and find it.

The final output is 3.

Question 7

The seventh question, new new Foo().getName(); is also an operator priority issue.

The final actual execution is:

new ((new Foo()).getName)();
First initialize the instantiated object of Foo, and then use the getName function on its prototype as the constructor new again.

The final result is 3

Finally

As far as answering questions is concerned, the first question can be answered correctly 100% of the time, the second question can only be answered correctly 50% of the time, the third question can be answered correctly not many, and the fourth question can be answered very, very rarely . In fact, there are not many tricky and bizarre uses for this question. They are all scenarios that you may encounter. Most people with 1 to 2 years of work experience should be completely correct.

I can only say that some people are too impatient and dismissive. I hope everyone can understand some features of js through this article.