// recap
let single = 'single-quoted';
let double = "double-quoted";
let backticks = `backticks`;
// backtick can put expression
function sum(a, b) {
return a + b;
}
alert(`1 + 2 = ${sum(1, 2)}.`); // 1 + 2 = 3.
// span multiple lines
let guestList = `Guests:
* John
* Pete
* Mary
`;
alert(guestList); // a list of guests, multiple lines
// error
let guestList = "Guests: // Error: Unexpected token ILLEGAL
* John";
Special characters
let guestList = "Guests:\n * John\n * Pete\n * Mary";
alert(guestList); // a multiline list of guests
alert( "Hello\nWorld" ); // two lines using a "newline symbol"
// two lines using a normal newline and backticks
alert( `Hello
World` );
let str = `Hello`;
// the first character
alert( str[0] ); // H
alert( str.charAt(0) ); // H
// the last character
alert( str[str.length - 1] ); // o
// 不同之處
let str = `Hello`;
alert( str[1000] ); // undefined
alert( str.charAt(1000) ); // '' (an empty string)
// 字串迴圈
for (let char of "Hello") {
alert(char); // H,e,l,l,o (char becomes "H", then "e", then "l" etc)
}
Strings are immutable
// 字串不能修改
let str = 'Hi';
str[0] = 'h'; // error
alert( str[0] ); // doesn't work
// 只能重新儲存新的值
let str = 'Hi';
str = 'h' + str[1]; // replace the string
alert( str ); // hi
let str = 'Widget with id';
alert( str.indexOf('Widget') ); // 0, because 'Widget' is found at the beginning
alert( str.indexOf('widget') ); // -1, not found, the search is case-sensitive
alert( str.indexOf("id") ); // 1, "id" is found at the position 1 (..idget with id)
let str = 'Widget with id';
alert( str.indexOf('id', 2) ) // 12
// use in loop
let str = 'As sly as a fox, as strong as an ox';
let target = 'as'; // let's look for it
let pos = 0;
while (true) {
let foundPos = str.indexOf(target, pos);
if (foundPos == -1) break;
alert( `Found at ${foundPos}` );
pos = foundPos + 1; // continue the search from the next position
}
// shorter
let str = "As sly as a fox, as strong as an ox";
let target = "as";
let pos = -1;
while ((pos = str.indexOf(target, pos + 1)) != -1) {
alert( pos );
}
// 不能直接當判斷式
let str = "Widget with id";
if (str.indexOf("Widget")) {
alert("We found it"); // doesn't work!
}
// 要換另一種寫法
let str = "Widget with id";
if (str.indexOf("Widget") != -1) {
alert("We found it"); // works now!
}
alert( ~2 ); // -3, the same as -(2+1)
alert( ~1 ); // -2, the same as -(1+1)
alert( ~0 ); // -1, the same as -(0+1)
alert( ~-1 ); // 0, the same as -(-1+1)
// 在老的程式碼使用這樣的方法當判斷式
let str = "Widget";
if (~str.indexOf("Widget")) {
alert( 'Found it!' ); // works
}
includes, startsWith, endsWith 檢查字串是否有子字串，返回布林值。
// includes
alert( "Widget with id".includes("Widget") ); // true
alert( "Hello".includes("Bye") ); // false
alert( "Midget".includes("id") ); // true
alert( "Midget".includes("id", 3) ); // false, from position 3 there is no "id"
// startsWith & endsWith
alert( "Widget".startsWith("Wid") ); // true, "Widget" starts with "Wid"
alert( "Widget".endsWith("get") ); // true, "Widget" ends with "get"
Getting a substring
str.slice(start [, end]) 返回開始點到結束點的部分字串
let str = "stringify";
alert( str.slice(0, 5) ); // 'strin', the substring from 0 to 5 (not including 5)
alert( str.slice(0, 1) ); // 's', from 0 to 1, but not including 1, so only character at 0
// 沒有第 2 個參數直接到結尾
let str = "stringify";
alert( str.slice(2) ); // ringify, from the 2nd position till the end
// 位置為負數也可以
let str = "stringify";
// start at the 4th position from the right, end at the 1st from the right
alert( str.slice(-4, -1) ); // gif
let str = "stringify";
// these are same for substring
alert( str.substring(2, 6) ); // "ring"
alert( str.substring(6, 2) ); // "ring"
// ...but not for slice:
alert( str.slice(2, 6) ); // "ring" (the same)
alert( str.slice(6, 2) ); // "" (an empty string)
str.substr(start [, length]) 返回從開始點計算長度的字串
let str = "stringify";
alert( str.substr(2, 4) ); // ring, from the 2nd position get 4 characters
let str = "stringify";
alert( str.substr(-4, 2) ); // gi, from the 4th position get 2 characters
alert( '𝒳'.length ); // 2, MATHEMATICAL SCRIPT CAPITAL X
alert( '😂'.length ); // 2, FACE WITH TEARS OF JOY
alert( ''.length ); // 2, a rare chinese hieroglyph
alert( '𝒳'[0] ); // 奇怪的符号...
alert( '𝒳'[1] ); // ...代理对的一块
// charCodeAt is not surrogate-pair aware, so it gives codes for parts
alert( '𝒳'.charCodeAt(0).toString(16) ); // d835, 在 0xd800 和 0xdbff 之间
alert( '𝒳'.charCodeAt(1).toString(16) ); // dcb3, 在 0xdc00 和 0xdfff 之间