Default Function Parameters.md
Default Function Parameters

function getSum(a,b){a = (a !== undefined) ? a : 1 ;b = (b !== undefined) ? b : 41;console.log(a+b); }getSum(); getSum(1,2); getSum (10); getSum(null, 6);

In ES5,if the argument is not specified,its value would be set to undefined.
if we do this,what will be happen?

function getSum(a, b) {a = 1;b = 41;console.log(a + b); } getSum(); getSum(1,2); getSum(10); getSum(null, 6);

if this?

function getSum(a, b) {console.log( a + b); } getSum(); getSum(1,2); getSum(10); getSum(null, 6);

//ES6 function getSum(a = 1, b = 41 ) {console.log(a + b); }getSum(); getSum(1, 2); getSum(10); getSum(null, 6);

In ES6 sets default values trying to streamline this process.

var getAnswer = function(number = 42, item = "universe"){console.log(number + " is the answer to " + item); } getAnswer(undefined, "life"); //42 is the answer to life. var getName = function(firstName = "John", lastName = "Doe") {console.log(firstName + " " + lastName); }getName("Jone"); //Jone Doe. var defaultName = "John"; var getName = function(firstName = defaultName, lastName = "Doe"){console.log(firstName + " " + lastName); }; getName(); //John Doe var getFirstName = () => "John";var getName = function( firstName = getFirstName(),lastName = "Doe"){console.log(firstName + " " + lastName); }getName(); //John Doe

How to check the numbers of the arguments?
we can check the numbers of the arguments by arguments.length.

var getName = function(firstName, lastName = "Doe"){console.log(arguments.length); } getName("John"); //1

Even thougn the second argument get a default value, arguments.length only returns the number of arguments passed to it.

var getPrice = function(quantity = price, price = 5){console.log(quantity + " ," + price); }getPrice();

This is a TDZ reference Error.
Because the parameters scope just between the parentheses(...).It isn't in a function body scope.
dynamic function

var getNumber = new Function("number = 42", "return number;"); console.log(getNumber());

summary

the type of default arguments
1.set a default value to the parameter in the function declaration statement itself.

2.function default values can be any valid expression.

3.function call

4.We can also access the other variables in the expression used as the default value.
Question what is dynamic function?

轉(zhuǎn)載于:https://www.cnblogs.com/InnerPeace-Hecdi/p/8870632.html

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