Statement vs Expression

The biggest difference between them

• Expression will return a value, but Statement won’t.

All category of statement

From MDN Document

• Control flow
• Declarations
• Functions and classes
• Iterations
• Others

Specifications

Statement

When declare a variable in JS, it will set up a space in memory.
for example

let a='apple';

But actually it can be separated into 2 steps.

let a; // statement, create a space in memory, and activate hoisting
a='apple'; // expression

Function Statement

Also called “Named Function”, example as below.
“apple” is the name of this function.

function apple(){
    console.log('banana');
};

When we create Function Statement, same as variable declaration, but
It also set up a space inside memory, and will also activate hoisting.

Function Expression

Also called “Anonymous Function”, example as below.

var apple=function (){
    console.log('orange');
};

But actually it can be separated into 2 steps.

var apple; // // statement, create a space in memory, and activate hoisting
apple=function ()
{console.log('orange');}; //// expression

Example 1

When we saw below code,

cat();
function cat(){
    console.log("貓咪");
};

the real execution sequence will be as below

 function cat(){
    console.log("貓咪");
};
cat();

Example 2

When we saw below code,

 var dog = function(){
    console.log("狗");
};
dog();

the real execution sequence will be as below

var dog;
dog= function(){
    console.log("狗");
};
dog();

Example 3

When we saw below code,

whosName()
function whosName() {
  if (myName) {
    myName = 'Jay';
  }
}
var myName = 'Ming';
console.log(myName);

the real execution sequence will be as below

function whosName() {
  if (myName) {
    myName = 'Jay';
  }
}
var myName;
whosName()
myName = 'Ming';
console.log(myName); // Ming