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How to operate the default parameters of processing functions in ES5 and ES6 environments
How to operate the default parameters of processing functions in ES5 and ES6 environments
This time I will show you how to operate the default parameters of the processing function in the ES5 and ES6 environments. What are the precautions for operating the default parameters of the processing functions in the ES5 and ES6 environments. The following is a practical case, let's take a look. take a look.
The default value of the function is something that greatly improves the robustness (that is, it makes the program more robust) MDN's description of the default parameters of the function: The default parameter of the function allows to be passed without a value or undefined Use the default parameters when entering.ES5
Use logical OR || to achieve
As we all know, in the ES5 version, there is no direct method for us to deal with the default value of the functionSo we can only enhance the function of the function ourselves, which is usually done like this:
function doSomething (name, age) {
name = name || 'default name'
age = age || 18
console.log(name, age)
}uses the default value.
After executing the function, there seems to be nothing wrong:doSomething() // default name, 18
doSomething('Niko') // Niko , 18
doSomething(, 12) // default name, 12doSomething('Niko', 0) // Niko, 18console.log(0 || 'wrong')
console.log('' || 'wrong')
console.log(null || 'wrong')
console.log(false || 'wrong')Correct posture
So, the correct default value processing in ES5 should be like this:function doSomething (name, age) {
if (name === undefined) {
name = 'default name'
}
if (age === undefined) {
age = 18
}
console.log(name, age)
}ternary operator operator to simplify the operation
Or we can shorten it to the ternary operator form:function doSomething (name, age) {
name = name === undefined ? 'default name' : name
age = age === undefined ? 18 : age
console.log(name, age)
}Use functions for encapsulation
But if we have to repeat these operations every time we write a functionIt would be too troublesome, so we This logic is simply encapsulated:
function defaultValue (val, defaultVal) {
return val === undefined ? defaultVal : val
}
function doSomething (name, age) {
name = defaultValue(name, 'default name')
age = defaultValue(age , 18)
console.log(name, age)
} we can use this method to save the operation of the ternary operator:
function defaultValue () {
return arguments[+(arguments[0] === undefined)]
}ES6
The default value of the function in the ES6 version is basically the routine we implemented aboveBut because It is native, so there will be corresponding new syntax, which can be used more concisely:function doSomething (name = 'default name', age = 18) {
console.log(name, age)
}Using this method directly eliminates the need to check the default value inside the function, allowing the function to focus on doing what it should do.
How to throw exceptions for certain required parameters
This new syntax of ES6 allows us to Error reminder for a required parameter:function requireParams () {
throw new Error('required params')
}
function doSomething (name = requireParams(), age = 18) {
// do something
}Default value processing of complex structure parameters
The above processing is for simple basic type data, but if we There is a function as follows:function init ({id, value}) {}
init({
id: 'tagId',
value: 1
})如果在ES5环境下，针对这种参数的默认值处理将会变得无比复杂
首先要判断这一个参数是否存在，然后在判断参数中的所有key是否存在
而在ES6中，可以这样来做：
function init ({
id = 'defaultId',
value = 1
} = {}) {
console.log(id, value)
}
init()首先在解构函数的后边添加默认值= {}，然后针对每一项参数添加默认值，很简洁的就实现了我们的需求。
ES5版本的polyfill代码在仓库中的位置：defaultValue
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