Javascript tutorial use strict strict mode detailed explanation

Strict mode: The JavaScript standard defined by the ECMA-262 specification has stronger restrictions on javascript. This article mainly introduces the detailed explanation of JavaScript strict mode use strict. Friends who need it can refer to

Strict mode: The JavaScript standard defined by the ECMA-262 specification has stronger restrictions on javascript.

(Non-strict mode is called "sloppy mode/sparse mode/lazy mode".)

1. Use of strict mode

Strict mode can be implemented at the script or function level. (i.e. global and local modes)

1. Global

Add "use strict" at the front of the js file

2. Local

in the function Add "use strict" internally, as follows

function fn() {
  "use strict";
  //some code
}

2. Comparison between strict mode and non-strict mode

1. In strict mode, variables cannot be deleted (delete is an unqualified identifier)). In non-strict mode, deletion failure will return false

"use strict";
var x;
delete x;//报错Delete of an unqualified identifier in strict mode.（不合格的标识符）

2. In strict mode, if the function parameter has the same name, an error will be thrown; non-strict mode will not

In strict mode

function fn(a,a){
 "use strict";
  result=a+a;
  console.log(result);
}
fn(2,4);//Duplicate parameter name not allowed in this context(重复的参数名称在此上下文中不允许)

In non-strict mode

function fn1(a,a){
  "use strict";
  result=a+a;
  console.log(result);
}
fn1(2,4);//结果为8

3. Strict mode does not allow octal integers Direct quantity (below). No error will be reported in non-strict mode.

"use strict"
var x=089;
console.log(x);//报错：Decimals with leading zeros are not allowed in strict mode.

4. In strict mode, the arguments object is a static copy of the actual parameter list passed into the function (that is, changes in parameters are no longer tracked); in non-strict mode, The elements in the arguments object and the corresponding actual parameters are references to the same value.

In strict mode

fn(5);
function fn(a){
  "use strict";
  a = 42;
  // return a==arguments[0];//返回false
  console.log(a);//结果为42
  console.log(arguments[0]);//结果为5,严格模式下arguments[0]表示这个调用方法的第一个参数     (不再追踪  参数的变化，在函数内部，参数a被重新赋值为42，但arguments[0]仍然为5.)
}

In non-strict mode

fn(5);
function fn(a){
  a = 42;
  // return a==arguments[0];//返回true
  console.log(a);//结果为42
  console.log(arguments[0]);//结果为42(追踪参数变化)

}

5. In strict mode eval and arguments are used as keywords, they cannot be assigned and used as variable declaration

"use strict";
var eval=3;//报错：Unexpected eval or arguments in strict mode
var argument=6；//同样报错

6. Strict mode will limit the ability to detect the call stack, access arguments.callee, arguments.callee.caller will throw an exception

In strict mode:

"use strict";
function fn(n){
  if(n==0){
    return 1;
  }else{
    return n*arguments.callee(n-1);
  }
}
console.log(fn(5));//报错

In non-strict mode:

function fn(n){//阶乘
  if(n==0){
    return 1;
  }else{
    return n*arguments.callee(n-1);
  }
}
console.log(fn(5));//正常120

caller: When a function calls another function, the called function will automatically generate a caller attribute pointing to the function object that called it. If the function is not currently called, or is not called by another function,

caller is null. As follows:

In strict mode:

"use strict"
function fn1() {
  var n1= fn1.caller;
  console.log(n1);
}
function fn2() {
  fn1();
}
fn2();//报错

In non-strict mode:

function fn1() {
  var n1= fn1.caller;//此例中caller指向调用它的函数（fn2）
  console.log(n1);
}
function fn2() {
  fn1();
}
fn2();// 结果打印出：function fn2(){ fn1( ) };

7 .Strict mode variables must be declared first and assigned directly to the variable. Global variables will not be created implicitly and with cannot be used.

In strict mode

"use strict"；
with（obj）{
  x；
}//报错：Strict mode code may not include a with statement

Explanation: When running a script, it is compiled first and then executed. Obviously at compile time, it cannot be determined at compile time which object the variables in the with statement block belong to. [Is the variable in the with statement block an attribute

of (cl) or a variable in the upper-level variable scope chain. ]. It can only be determined at execution time that (c1) is an instance of Clothes. This conflicts with strict mode, which checks whether variables are defined at compile time, so strict mode does not use the with statement.

Extension: Use of with statement (used to reference existing attributes in a specific object, but cannot be used to add attributes to the object.)

with(object instance)
{
  //代码块
}
eg：function Clothes(){
  this.color="red";
  this.size="m";
  this.price="cheap";
}
var c1=new Clothes();
with(c1){
  var str="颜色："+color+"，尺寸："+size+"，价格："+price;
  document.write(str);
}//结果(颜色：red，尺寸：m，价格：cheap)

In strict mode, null undefined passed in to call apply remains as is and is not converted to window

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