What is the difference between let and const

Difference: Variables declared by let can be changed, and both value and type can be changed; while constants declared by const cannot be changed, which means that once const is declared, it must be initialized immediately and cannot be assigned a value later.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

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The difference between let and const:

① The variables declared by let can be changed, and both the value and type can be changed; the constants declared by const cannot be changed, which means that once const is declared, it must be initialized immediately and cannot be assigned a value later.

const i ; // 报错，一旦声明，就必须立即初始化 const j = 5; j = 10; // 报错，常量不可以改变

②For composite type variables such as arrays and objects, the variable name does not point to the data, but to the address where the data is located. const only guarantees that the address pointed to by the variable name remains unchanged, but does not guarantee that the data at that address remains unchanged, so you must be very careful when declaring a composite type variable as a constant.

const arr = []; // 报错，[1,2,3]与[]不是同一个地址 arr = [1,2,3]; const arr = []; // 不报错，变量名arr指向的地址不变，只是数据改变 arr[0] = 1; arr[1] = 2; arr[2] = 3; console.log(arr.length); // 输出：3

If you want the data of the defined object or array not to change, you can use object.freeze(arr) to freeze. Freezing means that new attributes cannot be added to this object or array, the values of existing attributes cannot be modified, and existing attributes cannot be deleted.

const arr = []; Object.freeze(arr); // 不报错，但数据改变无效 arr[0] = 1; arr[1] = 2; arr[2] = 3; console.log(arr.length); // 输出：0

The similarities between let and const:

① It is only valid within the block-level scope where it is declared.

② No promotion, and there is a temporary dead zone, which can only be used after the declared position.

③ Cannot be repeated.

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