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Differences: 1, let declaration is a variable, its value and type can be changed, while const declaration is constant, can not be changed, can not be re-assigned; 2, let variable, the declaration can not be initialized, and const constant, once declared, it must be initialized immediately; 3, const always points to a fixed address, and let is not fixed.

The operating environment of this tutorial: windows7 system, ECMAScript version 6, Dell G3 computer.

Let and const are two new ways to declare variables in es6, although we can still use the widely read var variable, but now we have two better tools to use: let and const.

The difference between let and const in es6

① let declares variables whose values and types can be changed; const declares constants whose values and types cannot be changed.

The ② let variable can be uninitialized after declaration, while the const constant, once declared, must be initialized immediately and cannot be assigned later.

Const I; / / error, once declared, must immediately initialize const j = 5 * * j = 10; / / error, constant cannot be changed

③ const always points to a fixed address, while let is not fixed

Const foo = {}; / / add an attribute to foo. If you can successfully foo.prop = 123 position foo.prop / / 123 / / point the foo to another object, it will report an error foo = {}; / / TypeError: "foo" is read-only

In the above code, the constant foo stores an address that points to an object. What is immutable is this address, that is, you cannot point the foo to another address, but the object itself is mutable, so you can still add new properties to it.

Description:

For variables of compound types, 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 the same, not the data at that address, so you must be very careful when declaring a variable of a compound type as a constant.

Const arr = []; / / error is not the same address as [1Jing 2jue 3]; const arr = []; / / No error is reported, the address pointed to by the variable name arr remains the same, except that the data changes arr [0] = 1 Arr [1] = 2 Arr [2] = 3 position console.log (arr.length); / / output: 3

Similarities between let and const in es6:

① is valid only within the block-level scope in which the declaration is located.

There is no declaration promotion in ②, and there is a temporary dead zone, which can only be used after the declared location.

Console.log (dadi); let dadi = 569

Result: will report an error

③ cannot repeat the declaration.

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