(1) +号方式

var htmlSTring = "<div>"+
    "this is a string"+
"</div>";

(2) \方式

var htmlSTring="<div>\
    This is a string.\
</div>"

(3) es6方式

var htmlSTring = `<div>
   This is a string.
</div>`;

es6传变量方式

var b = "hello world"
var htmlSTring = `<div>
   This is a string: ${b}.
</div>`;

对比总结：

es6方式简洁不容易出错，打印结果有所差别， \ 方式会保留一些空格，es6的字符串模板 ` (在esc键下方)方式会保留换行，还支持传变量

参考：Multiline String Variables in JavaScript