Example tutorial of function extension in ES6

Some new features have also been added in the direction of function expansion. I feel that these features are also very important.

1. Parameter default values ​​(note: the default value cannot be followed by Add parameters without default values)

{  function test(x, y = 'world'){
    console.log('默认值',x,y);
  }
  test('hello');//hello world
  test('hello','kill');//hello kill
}

{
  let x='test';  function test2(x,y=x){
    console.log('作用域',x,y);
  }
  test2('kill');//kill kill  这里涉及到作用域的问题   函数里面具有单独的作用域  只有没有x的时候 才会继承let所声明的x
}

　2. Rest parameters (...) Convert a series of discrete values ​​into Array Similarly, rest cannot be followed by any parameters

{  function test3(...arg){for(let v of arg){
      console.log('rest',v);
    }
  }
  test3(1,2,3,4,'a');
}

　3. The spread operator (...) converts an array into a series of discrete values 　

{
  console.log(...[1,2,4]);
  console.log('a',...[1,2,4]);
}

　4. Arrow function (very important, otherwise some new codes cannot be understood!!!) For example, a=> ;a*2 a is the parameter a*2 is the return value => as a symbol of the function When no parameters are passed, () can be used to represent

{
  let arrow = v => v*2;
  let arrow2 = () => 5;
  console.log('arrow',arrow(3));//6
  console.log(arrow2());//5

}

　5. Tail call If a function nests another function, you can consider tail call

{  function tail(x){
    console.log('tail',x);
  }  function fx(x){return tail(x)
  }
  fx(123)// tail 123
}

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