Javascript String

Creating strings

let str1 = 'abc';
let str2 = "abc";

escaping quotes in string

let str3 = 'I\'ve got';  # by putting a backslash just before the character.

Escape sequence  转义序列
\0 \' \" \\ \n  \r  \v  \t  \b  \f  
\uXXXX     : \u000A = \n, \u0021 = !
\u{X}...\u{XXXX} 0000-FFFF  
\xXX 00 -FF

joining strings together
连接字符串， use  plus + operator， you can join as many as you like, as long as you include a + between each pair. 


const button = document.querySelector('button');

button.onclick = function() {
  let name = prompt('What is your name?');
  alert('Hello ' + name + ', nice to see you!');
}

用字符串表示为一个数字不行，但是用数字表示一个字符串可以。

'Front ' + 242;

let myString = '123';
let myNum = Number(myString);
typeof myNum;

let myNum2 = 123;
let myString2 = myNum2.toString();
typeof myString2;



~~~~~~~~~~~~~~~~~~~~~~~~~~~
HTML provides structure and meaning to our text.
CSS allows us to precisely style it.
JavaScript contains a number of features for manipulating strings, creating custom welcome messages and prompts, showing the right text labels when needed, sorting terms into the desired order, and much more.

9:00
Template literals 模板文本

把字符串变成模板，你需要 replace the quote marks with backtick characters （``）

let song = 'Fight the Youth';
Would be turned into a template literal like so:

song = `Fight the Youth`;

If we want to concatenate strings, or include expression results inside them, traditional strings can be fiddly to write:

let score = 9;
let highestScore = 10;
let output = 'I like the song "' + song + '". I gave it a score of ' + (score/highestScore * 100) + '%.';

Template literals simplify this enormously:模板文本极大地简化了这一点：

output = `I like the song "${ song }". I gave it a score of ${ score/highestScore * 100 }%.`;

When you want to include a variable or expression inside the string, you include it inside a ${ } construct, which is called a placeholder.

let examScore = 45;
let examHighestScore = 70;
examReport = `You scored ${ examScore }/${ examHighestScore } (${ Math.round(examScore/examHighestScore*100) }%). ${ examScore >= 49 ? 'Well done, you passed!' : 'Bad luck, you didn\'t pass this time.' }`;

Template literals respect the line breaks in the source code, so newline characters are no longer needed. This would achieve the same result:

output = `I like the song "${ song }".
I gave it a score of ${ score/highestScore * 100 }%.`;

let quote = 'I do not like green eggs and ham. I do not like them, Sam-I-Am.';
let substring = 'green eggs and ham';

// Add your code here
let  quoteLength = quote.length;
let index = quote.indexOf(substring);
let revisedQuote = `I do not like ${substring}.`;
let revisedQuote = `${quote.substr(0,index)}${substring}.`;
// Don't edit the code below here!

section.innerHTML = ' ';
let para1 = document.createElement('p');
para1.textContent = `The quote is ${ quoteLength } characters long.`;
let para2 = document.createElement('p');
para2.textContent = revisedQuote;

section.appendChild(para1);
section.appendChild(para2);


indexOf('hello'); replace('a','b'); toLowerCase();substr(0,1)


(5 ** 2 + 8 ** 2) ** 0.5
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