[string1, num, string2].join(' ');

['string1', num, 'string2'].join(' ');

var num = 100;
var c = "string1 "+num+" string2";

或者

var num = 100;
var c = `string1 ${num} string2`;

楼主你写的是对的，只是字符串的写法有问题

var num = 100;
console.log('string1 '+num+' string2')

这个就是你要的结果

var num = 100;
var c = "string1 '+num+' string2";
//字符串+数字+字符串是没有问题的只是"string'一个单引号一个双引号是错误的
var c = "string1 "+num" string2";
//或者
var c = 'string1 '+num+' string2';

var num = 100
var c = 'string1 '+num+' string2' //string1 100 string2

es6:

var num = 100
var c = `string1 ${num} string2` //string1 100 string2