Some of the problems ES6 block-level scopes solve

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Introduction to the

There was no block-level scope in ES5, which caused a lot of problems, but in ES6 there was a new block-level scope

Problem 1: Inner variables overwrite outer variables

Due to the variable declared by var, there is a variable boost, with the inner TMP overwriting the outer TMP variable

var tmp = 'hahaha';

function f() {
  if (false) {
    var tmp = 'hello world';

f(); // undefined
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Problem 2: Loop variables in a for loop leak as global variables

With a for loop, the I variable is not recycled after the loop ends, but leaks out as a global variable

var s = 'hahaha';

for (var i = 0; i < s.length; i++) {

console.log(i); / / 6
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ES6 block-level scope

Let and const add block-level scope to JavaScript

The outer code block is not affected by the inner code block within the same block level

function f1() {
  let n = 5;
  if (true) {
    let n = 10;
  console.log(n); / / 5
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Function declaration

ES5 states that functions can only be declared in top-level scopes and function scopes, not block-level scopes. However, browsers do not comply with this rule and, for compatibility with older code, still support declaring functions in block-level scopes, so both cases actually work without error.

Code like the following is illegal in ES5, but will not report errors in the browser

/ / a
if (true) {
  function f() {}}/ / 2
try {
  function f() {}}catch(e) {
  // ...
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ES6 solves this problem by introducing block-level scopes, which explicitly allow functions to be declared in block-level scopes.

ES6 states that a function declaration statement in a block-level scope behaves like a LET and cannot be referenced outside the block-level scope.

However, such changes are very unfriendly to old code, and to mitigate the resulting incompatibilities, ES6 provides in Appendix B that browser implementations can behave in their own way.

  • Allows functions to be declared in block-level scopes.
  • The function declaration looks something likevarIs promoted to the head of the global scope or function scope.
  • Function declarations are also promoted to the head of their block-level scope.

Note that the above three rules apply only to browser implementations of ES6. Implementations of other environments do not follow these rules, and block-scoped function declarations are treated as lets.

Function expressions and function declarations

You should avoid declaring functions in block-level scopes because of the wide variation in behavior caused by the environment. If necessary, write it as a function expression rather than a function declaration statement.

// Function declaration statements inside the block-level scope are not recommended
  let a = 'secret';
  function f() {
    returna; }}// Inside the block-level scope, function expressions are preferred
  let a = 'secret';
  let f = function () {
    return a;
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The identity of the scope

ES6 block-level scopes must have braces, or identifiers, and if there are no braces, the JavaScript engine assumes that there is no block-level scope.

if (true) let x = 1;  / / an error

if (true) {
  let x = 1;
/ / is not an error
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