[TS] function type

One 、 Function type

• Specify the function parameter type

//  Function declaration （Function Declaration）
function sum(x:number, y:number) {
    
    return x + y;
}
sum(1,2)	//  correct 
sum('1',2)	//  Report errors ,x、y Type must be set number type 

//  Function expression （Function Expression）
let mySum = function (x:number, y:number) {
    
    return x + y;
};
mySum(1,2)		//  correct 
mySum('1',2)	//  Report errors ,x、y Type must be set number type 

• Specifies the return value type of the function

//  Function declaration （Function Declaration）
function sum(x, y):number {
    
    return x + y;
}
sum(1,2)	//  correct 
sum('1',2)	//  Report errors ,'1',2  String splicing , So the return value is string, It's not set up number type 

//  Function expression （Function Expression）
let mySum = function (x, y):number {
    
    return x + y;
};
mySum(1,2)		//  correct 
mySum('1',2)	//  Report errors ,'1',2  String splicing , So the return value is string, It's not set up number type 

Two 、 Parameter setting

2.1、 Optional parameters

• Type in the extra （ Or less than required ） Parameters , It's not allowed . So how to define the optional parameters ？
• We use it ? Represents an optional parameter

function buildName(firstName: string, lastName?: string) {
    
    if (lastName) {
    
        return firstName + ' ' + lastName;
    } else {
    
        return firstName;
    }
}
let tomcat = buildName('Tom', 'Cat');	//  Print the results ：'Tom Cat'
let tom = buildName('Tom');				//  Print the results ：'Tom '

• It should be noted that , Optional parameters must be followed by required parameters

function buildName(firstName?: string, lastName: string) {
    
    if (firstName) {
    
        return firstName + ' ' + lastName;
    } else {
    
        return lastName;
    }
}
let tomcat = buildName('Tom', 'Cat');
let tom = buildName(undefined, 'Tom');

// index.ts(1,40): error TS1016: A required parameter cannot follow an optional parameter.
//  translate ： error TS1016: A required parameter cannot follow an optional parameter 

2.2、 Default parameters

• stay ES6 in , We allow you to add default values to the parameters of a function .
• and TypeScript Parameters with added default values are recognized as optional parameters ：

function buildName(firstName: string, lastName: string = 'Cat') {
    
    return firstName + ' ' + lastName;
}
let tomcat = buildName('Tom', 'Caa');	//  Print the results ：'Tom Caa'
let tom = buildName('Tom');				//  Print the results ：'Tom Cat'

• But at this point 「 Optional parameters must be followed by required parameters 」 The limit of ：

function buildName(firstName: string = 'Tom', lastName: string) {
    
    return firstName + ' ' + lastName;
}
let tomcat = buildName('Tom', 'Cat');	//  Print the results ：'Tom Cat'
let cat = buildName(undefined, 'Cat');	//  Print the results ：'Tom Cat'

2.3、 The remaining parameters

• ES6 in , have access to ...rest Get the remaining parameters in the function （rest Parameters ）：

function push(array, ...items) {
    
    items.forEach(function(item) {
    
        array.push(item);
    });
}

let a = [];
push(a, 1, 2, 3);
console.log(a)	// [1, 2, 3]

• in fact ,items Is an array . So we can define an array by its type ：

function push(array: any[], ...items: any[]) {
    
    items.forEach(function(item) {
    
        array.push(item);
    });
}

let a = [];
push(a, 1, 2, 3);

• Be careful ：rest The remaining parameter can only be the last parameter