The difference between a JavaScript function declaration and a function expression

There are several ways to define a function, the most common being the function declaration and the function expression, although you can implement the definition function, but there is a difference between them.

1. For example:

1.1 Function declarations

function log () {
     Console.log.apply (console, arguments);
}

Log is the function name that points to the function object it declares.

1.2 Function expressions

var function () {      console.log.apply (console, arguments);};

Log also points to the function object it declares, but as a variable.

1.3 from the subsequent call, they can all achieve the same effect, no difference. But maybe you accidentally called before the function definition, and the difference came.

1.3.1 is called after the function definition, the effect is the same.

function log () {      console.log.apply (console, arguments);} var function () {      console.log.apply (console, arguments);}; Log (1); log2 (2);

1.3.2 called before the function definition, LOG2 calls an error.

Log (1); log2 (2); function log () {      console.log.apply (console, arguments);} var function () {      console.log.apply (console, arguments);};

1.3.3 The reason is that the parser reads the two definitions in a different order, the parser reads the function declaration in advance and makes it available before executing the subsequent code, whereas for a function expression, the parser executes only when it reads the line where the function expression is located, simple, like this:

var a = b; var b=0; Console.log (a);

At this point, log out of a is undefined, at the time of assignment A, the parser does not read to B.

The difference between a JavaScript function declaration and a function expression