Thinking caused by the assignment of the object often asked in the interview

1.

var a = 1;
var b = a;
a = 2;
console.log(b); // 1

var c = [1,2,3];
var d = c;
c.shift();
console.log(d); // [2,3]

We know that the assignment of the basic object is a copy value, so changing a does not affect b, while the complex object assignment copies the pointer, so changing c affects d

2.
but now there are some strange phenomena

var a = [1,2,3];
var b = a;
a = [1,2,3,4];
console.log(b); // [1,2,3]

changed a but b didn"t change here. I don"t know what happened in the middle. Did I open up an address to store a [1mage2jin3jin4]? B or point to the previous a?

3.

var factorial = function() {
  return 1
};
var trueFactorial = factorial;
factorial = function () {
  return 2;
}
console.log(trueFactorial()); // 1

it"s strange, isn"t it the re-assignment of factorial? Isn"t the assignment of function a shallow copy?

4.

var factorial = function() {
  return factorial();
};
var trueFactorial = factorial;
factorial = function () {
  return 2;
}
console.log(trueFactorial()); // 2

Ah, the function that can be called normally to the second assignment here

Mar.21,2021

2, a = [1 code > creates a new array object, and then points a to it
3, var trueFactorial = factorial; assigns the object pointed to by factorial to trueFactorial , so trueFactorial points to the originally declared method


.

compound types (Function, Object, etc.) are assigned reference passing , which all point to the same memory address,

{
    "type": "Program",
    "start": 0,
    "end": 41,
    "body": [
        {
            "type": "VariableDeclaration",
            "start": 0,
            "end": 16,
            "declarations": [
                {
                    "type": "VariableDeclarator",
                    "start": 4,
                    "end": 15,
                    "id": { "type": "Identifier", "start": 4, "end": 5, "name": "a" },
                    "init": {
                        "type": "ArrayExpression",
                        "start": 8,
                        "end": 15,
                        "elements": [
                            { "type": "Literal", "start": 9, "end": 10, "value": 1, "raw": "1" },
                            { "type": "Literal", "start": 11, "end": 12, "value": 2, "raw": "2" },
                            { "type": "Literal", "start": 13, "end": 14, "value": 3, "raw": "3" }
                        ]
                    }
                }
            ],
            "kind": "var"
        },
        {
            "type": "VariableDeclaration",
            "start": 17,
            "end": 27,
            "declarations": [
                {
                    "type": "VariableDeclarator",
                    "start": 21,
                    "end": 26,
                    "id": { "type": "Identifier", "start": 21, "end": 22, "name": "b" },
                    "init": { "type": "Identifier", "start": 25, "end": 26, "name": "a" }
                }
            ],
            "kind": "var"
        },
        {
            "type": "ExpressionStatement",
            "start": 28,
            "end": 41,
            "expression": {
                "type": "AssignmentExpression",
                "start": 28,
                "end": 41,
                "operator": "=",
                "left": { "type": "Identifier", "start": 28, "end": 29, "name": "a" },
                "right": {
                    "type": "ArrayExpression",
                    "start": 32,
                    "end": 41,
                    "elements": [
                        { "type": "Literal", "start": 33, "end": 34, "value": 1, "raw": "1" },
                        { "type": "Literal", "start": 35, "end": 36, "value": 2, "raw": "2" },
                        { "type": "Literal", "start": 37, "end": 38, "value": 3, "raw": "3" },
                        { "type": "Literal", "start": 39, "end": 40, "value": 4, "raw": "4" }
                    ]
                }
            }
        }
    ],
    "sourceType": "script"
}

this is the program you want. Please distinguish what the assignment statement does. The assignment statement changes the value of Identifier. The pop statement is a member method and does not change the direction of the Identifer.

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