According to the Function. The prototype. The call () – the definition of JavaScript | MDN

The call() method calls a function with a specified value of this and one or more arguments given separately.

preface

This article only discusses changing the reference to this within normal functions, and to simplify the problem does not distinguish between strict mode /null/undefined

The default target type is object. No other types are discussed

Several ways to change the function this

Function assigns a value directly to the target object

Function.prototype.call2 = function (context, ... args) {
  let uniqueKey = Symbol("Globally unique reference");
  context[uniqueKey] = this;
  letresult = context[uniqueKey](... args);delete context[uniqueKey];
  return result;
};
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Disadvantages: If the object is frozen or is a Proxy, the function assignment fails and the original object structure is destroyed

function test(){
  return this.a+this.b
}
ob1 = {a:1.b:2}
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Call example

1 / / examples
Object.seal(ob1)
test.call2(ob1)
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2 / / examples
Object.freeze(ob1)
test.call2(ob1)
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3 / / examples
ob1 = new Proxy(ob1, {
  set(target, key, value){}}); test.call2(ob1)Copy the code

Will get the following error:

VM86:4 Uncaught TypeError: context[uniqueKey] is not a function
    at Function.call2 (<anonymous>:4:34)
    at <anonymous>:1:6
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Change this with the prototype chain

Function.prototype.call3 = function (context, ... args) {
  const obj = Object.create(context);
  let uniqueKey = Symbol("Globally unique reference");
  obj[uniqueKey] = this;
  returncontext[uniqueKey](... args); };Copy the code

This approach solves the problem of the object being frozen/property intercepted, but the calling object is not the original object

If there is an operation on the prototype, the unexpected behavior occurs

conclusion

Function. The prototype. The call is native code

A complete function.prototype. call cannot be implemented in pure JS