JS Elevation 7. Function Expressions (1)

There are two common ways to define functions:

1. function declaration

2. Function expressions

# differences

(1) function declaration, which has the feature of function declaration promotion.

(2) The Name property of the function declaration function is the names of the functions; functions defined with function expressions in ES5, the Name property is an empty string, and the name of the function in ES6.

# function Declaration Promotion

function declaration

1 SayHello (); // calling a function before the function declaration does not cause an error because the function declaration is promoted.  2function  SayHello () {3     console.log ("hello!"); // hello! 4     Console.log (Sayhello.name); // SayHello 5 }

function expression

 1  SayHello (); //  Typeerror:sayhello is not a function uses the method of a function expression to define functions, and calling a function before a variable declares a function will cause an error.  2  var  SayHello = function   () { 3  Console.log ("hello!"); // hello!  4  console.log (sayhello.name); // sayhello. Note: In ES5, the Name property of the anonymous function is an empty string (P176), which returns the original name of the named function in ES6.  5  }   6  SayHello (); //  

# Note the following situation

1 //don't do that. Because of the reason for the function declaration promotion, the following code is invalid in ECMAScript, and the browser tries to fix the error in an inconsistent manner. P1762  varFlag =true;3  if(flag) {4      functionSayhi () {5Console.log ("Hi");6      }7}Else{8      functionSayhi () {9Console.log ("Yo");Ten      } One  } ASayhi ();//Strict mode will error. Referenceerror:sayhi is not defined

1 //You can do this by using a function expression. 2  varFlag =true;3  varSayhi;4  if(flag) {5Sayhi =function(){6Console.log ("Hi");7      }8}Else{9Sayhi =function(){TenConsole.log ("Yo"); One      } A  } -Sayhi ();

JS Elevation 7. Function Expressions (1)