js if条件判定 早見表

// 値
var v;
var v = '';
var v = 0;
var v = '0';
var v = null;
var v = 'null';
var v = undefined;
var v = 'undefined';
var v = [];

// console
console.log('v:' + v);

条件が空文字

値	console	if(v)	if(v == '')	if(v === '')	if(!v)	if(v != '')	if(v !== '')
v	value:undefined	false	false	false	true	true	true
v = ''	value:	false	true	true	true	false	false
v = 0	value:0	false	true	false	true	false	true
v = '0'	value:0	true	false	false	false	true	true
v = null	value:null	false	false	false	true	true	true
v = 'null'	value:null	true	false	false	false	true	true
v = undefined	value:undefined	false	false	false	true	true	true
v = 'undefined'	value:undefined	true	false	false	false	true	true
v = []	value:	true	true	false	false	false	true

条件がnull

値	console	if(v == null)	if(v === null)	if(v != null)	if(v !== null)
v	value:undefined	true	false	false	true
v = ''	value:	false	false	true	true
v = 0	value:0	false	false	true	true
v = '0'	value:0	false	false	true	true
v = null	value:null	true	true	false	false
v = 'null'	value:null	false	false	true	true
v = undefined	value:undefined	true	false	false	true
v = 'undefined'	value:undefined	false	false	true	true
v = []	value:	false	false	true	true

条件がundefined

値	console	if(v == undefined)	if(v === undefined)	if(v != undefined)	if(v !== undefined)
v	value:undefined	true	true	false	false
v = ''	value:	false	false	true	true
v = 0	value:0	false	false	true	true
v = '0'	value:0	false	false	true	true
v = null	value:null	true	false	false	true
v = 'null'	value:null	false	false	true	true
v = undefined	value:undefined	true	true	false	false
v = 'undefined'	value:undefined	false	false	true	true
v = []	value:	false	false	true	true