Compilation of common JavaScript error-prone knowledge points

1. Variable scope

var a = 1; 
function test() { 
    var a = 2; 
    console.log(a); // 2 
} 
test();

A is declared and assigned in the function scope above, and it is above the console, so the output a is equal to 2 following the proximity principle.

var a = 1;  
function test2() { 
    console.log(a); // undefined 
    var a = 2; 
} 
test2();

Although a is declared and assigned in the function scope above, it is located under the console, and the a variable is promoted. It has been declared but has not been assigned a value during output, so "undefined" is output.

var a = 1; 
function test3() { 
    console.log(a); // 1 
    a = 2;  
} 
test3();

In the function scope above, a is reassigned, not re-declared, and is located under the console, so a in the global scope is output.

let b = 1; 
function test4() { 
    console.log(b); // b is not defined 
    let b = 2; 
} 
test4();

The ES6 let is used in the function scope above to redeclare the variable b. However, unlike var, let does not have the function of variable promotion, so the output error "b is not defined" is reported.

function test5() { 
    let a = 1; 
    { 
        let a = 2; 
    } 
    console.log(a); // 1 
} 
test5();

The function scope above uses let to declare a as 1, and declares a as 2 in the block-level scope. Because the console is not in the block-level scope within the function, 1 is output.

2. Type comparison

var arr = [], 
    arr2 = [1]; 
console.log(arr === arr2); // false

Compare two different arrays above, console is false.

var arr = [], 
    arr2 = []; 
console.log(arr === arr2); // false

Comparison of the two identical arrays above, because two separate arrays are never equal, so the console is false.

var arr = [], 
    arr2 = {}; 
console.log(typeof(arr) === typeof(arr2)); // true

The above uses typeof to compare arrays and objects. Because typeof obtains NULL, arrays, and objects, the types are all object, so the console is true.

var arr = []; 
console.log(arr instanceof Object); // true 
console.log(arr instanceof Array); // true

The above uses instanceof to determine whether a variable belongs to an instance of an object. Because arrays are also a type of object in JavaScript, both consoles are true.

3.this points to

var obj = { 
    name: 'xiaoming', 
    getName: function () { 
        return this.name 
    } 
}; 
console.log(obj.getName());  // 'xiaoming'

This in the object method above points to the object itself, so "xiaoming" is output.

var obj = { 
    myName: 'xiaoming', 
    getName: function () { 
        return this.myName 
    } 
}; 
var nameFn = obj.getName; 
console.log(nameFn()); // undefined

The method in the object is assigned to a variable above. At this time, this in the method will no longer point to the obj object, but to the window object, so the console is "undefined".

var obj = { 
    myName: 'xiaoming', 
    getName: function () { 
        return this.myName 
    } 
};  
var obj2 = { 
    myName: 'xiaohua'  
}; 
var nameFn = obj.getName; 
console.log(nameFn.apply(obj2)); // 'xiaohua'

The method in the obj object is also assigned to the variable nameFn above, but this is pointed to the obj2 object through the apply method, so the final console is 'xiaohua'.

4. Function parameters

function test6() { 
    console.log(arguments); // [1, 2] 
} 
test6(1, 2);

The above uses the arguments object in the function to obtain the parameter array passed into the function, so the output array is [1, 2].

function test7 () { 
    return function () { 
        console.log(arguments); // 未执行到此，无输出 
    } 
} 
test7(1, 2);

The above also uses arguments to obtain parameters, but because test7(1, 2) does not execute the function in return, there is no output. If test7(1, 2)(3, 4) is executed, [3, 4] will be output .

var args = [1, 2]; 
function test9() { 
    console.log(arguments); // [1, 2, 3, 4] 
} 
Array.prototype.push.call(args, 3, 4); 
test9(...args);

The above uses the Array.prototype.push.call() method to insert 3 and 4 into the args array, and uses the ES6 extension operator (...) to expand the array and pass it to test9, so the console is [1, 2, 3, 4].

5. Closure problem

var elem = document.getElementsByTagName('div'); // 如果页面上有5个div 
for(var i = 0; i < elem.length; i++) { 
    elem[i].onclick = function () { 
        alert(i); // 总是5 
    }; 
}

The above is a very common closure problem. The value that pops up when you click on any div is always 5, because when you trigger the click event, the value of i is already 5. You can use the following method Solution:

var elem = document.getElementsByTagName('div'); // 如果页面上有5个div 
for(var i = 0; i < elem.length; i++) { 
    (function (w) { 
        elem[w].onclick = function () { 
            alert(w); // 依次为0,1,2,3,4 
        }; 
    })(i); 
}

Encapsulate an immediate execution function outside the bound click event and pass i into the function.

6. Object copying and assignment

var obj = { 
    name: 'xiaoming', 
    age: 23 
}; 
 
var newObj = obj; 
newObj.name = 'xiaohua'; 
console.log(obj.name); // 'xiaohua' 
console.log(newObj.name); // 'xiaohua'

Above we assigned the obj object to the newObj object, thus changing the name attribute of newObj, but the name attribute of the obj object was also tampered with. This is because the newObj object actually obtained It's just a memory address, not a real copy, so the obj object is tampered with.

var obj2 = { 
    name: 'xiaoming', 
    age: 23 
}; 
var newObj2 = Object.assign({}, obj2, {color: 'blue'}); 
newObj2.name = 'xiaohua'; 
console.log(obj2.name); // 'xiaoming' 
console.log(newObj2.name); // 'xiaohua' 
console.log(newObj2.color); // 'blue'

Using the Object.assign() method above to perform a deep copy of the object can avoid the possibility of the source object being tampered with. Because the Object.assign() method can copy any number of the source object's own enumerable properties to the target object, and then return the target object.

var obj3 = { 
    name: 'xiaoming', 
    age: 23 
}; 
var newObj3 = Object.create(obj3); 
newObj3.name = 'xiaohua'; 
console.log(obj3.name); // 'xiaoming' 
console.log(newObj3.name); // 'xiaohua'

We can also use the Object.create() method to copy the object. The Object.create() method can create a new object with the specified prototype object and properties.

Conclusion

Learning JavaScript is a long process and cannot be accomplished overnight. I hope that the points introduced in this article can help students learning JavaScript to have a deeper understanding and mastery of JavaScript syntax and avoid detours.