Many times we want to create very long strings, such as when adding HTML dynamically. In PHP we can use

nowdoc (single quote) delimiter string: does not recognize variables, does not recognize escape characters (except \' and \\, which represent single quotes and backslashes, respectively)
heredoc (double quote) delimiter string: identify variables, identify escape characters

to create multiline strings. But how to do it in JS?

One: connect directly with the plus sign

var ts = '<!DOCTYPE html>'+
'<html lang="en">'+
'<head>'+
'  <meta charset="UTF-8">'+
'  <title>Document</title>'+
'</head>'+
'<body>'+
'  hello world'+
'</body>'+
'</html>'

Second, use the backslash (continuation character)

var ts = '<!DOCTYPE html>\
<html lang="en">\
<head>\
  <meta charset="UTF-8">\
  <title>Document</title>\
</head>\
<body>\
  hello world\
</body>\
</html>'

3. String array join

var ts = ['<!DOCTYPE html>',
'<html lang="en">',
'<head>',
'  <meta charset="UTF-8">',
'  <title>Document</title>',
'</head>',
'<body>',
'  hello world',
'</body>',
'</html>'].join('\n')

四、String.prototype.concat

var ts = String.prototype.concat.call('<!DOCTYPE html>',
'<html lang="en">',
'<head>',
'  <meta charset="UTF-8">',
'  <title>Document</title>',
'</head>',
'<body>',
'  hello world',
'</body>',
'</html>')

五、Function.prototype.toString()

==Note: This method ensures that JS is not compressed to remove comments, and pay attention not to remove the text when removing the comment number. ==

var ts = (function(){/*
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
</head>
<body>
  hello world
</body>
</html>
*/}).toString().split('\n').slice(1,-1).join('\n') + '\n'

Can be encapsulated as a method:

function doc(fn) {
  return fn.toString().split('\n').slice(1,-1).join('\n') + '\n'
}
var ts = doc(function(){/*
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
</head>
<body>
  hello world
</body>
</html>
*/})

六、CoffeeScript

To compile to get the JS file

ts = """
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
</head>
<body>
  hello world
</body>
</html>
"""

Seven, template strings (ES6 new features)

== is not supported by IE! ! ! is backticks "`" instead of single quotes "'" ==
template strings - JavaScript | MDN

var ts = `<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
</head>
<body>
  hello world
</body>
</html>`

JS - create multiline string