官术网_书友最值得收藏!

JavaScript rest and spread syntax

In JavaScript, a rest parameter collects multiple arguments and condenses them into a single argument. It is called rest because it collects the rest of the arguments into a single argument.

A rest parameter has nothing to do with Representational state transfer protocol (REST ).

This syntax was introduced in ES6 and allows us to nicely implement functions that have an indefinite number of parameters.

We define a rest parameter with three dots preceding the parameter name.

Let's go through a quick example:

  1. Let's create a logScores function that takes in a scores rest parameter that just outputs the parameter to the console:
function logScores(...scores) {
console.log(scores);
}
This is pure JavaScript - we'll introduce types to rest parameters when we look at the new features in TypeScript 3.
  1. We can call logScores as follows:
logScores(50, 85, 75);

If we run this, we'll get an array of the three elements we passed in as parameters output to the console. So, our scores parameter has collected all the arguments into an array.

The spread syntax is the opposite of rest parameters. It allows an iterable, such as array, to be expanded into function arguments.

Let's look at an example:

  1. Let's redefine our logScore function with specific parameters:
function logScore(score1, score2, score3) {
console.log(score1, score2, score3);
}

  Note that this is still pure JavaScript – no types just yet! 

  1. Let's define a scores array:
const scores = [75, 65, 80];
  1. Finally, let's use the spread syntax to pass our scores variable into our logScore function:
logScore(...scores);

If you are using the TypeScript playground, you'll get the compilation error, expected 3 arguments, but got 0 or moreThe program still runs though, because this is perfectly valid JavaScript. 75, 65, 80 will be output to the console if we do run it.

In the following sections, we'll see how the new features in TypeScript 3 help us help the compiler to better understand what we are trying to do when using rest and spread. This will allow us to resolve the compilation errors seen in the preceding example.

主站蜘蛛池模板: 武汉市| 闻喜县| 海城市| 柳州市| 长丰县| 繁昌县| 阿合奇县| 崇礼县| 沁源县| 奉节县| 苏尼特左旗| 杭锦旗| 天水市| 金沙县| 永安市| 龙胜| 西丰县| 增城市| 富蕴县| 湟源县| 抚顺市| 曲周县| 确山县| 什邡市| 苗栗市| 阿克| 宁河县| 金寨县| 奉节县| 康定县| 平武县| 汕尾市| 红原县| 浦县| 宁乡县| 凌云县| 广灵县| 金华市| 方正县| 固镇县| 景谷|