refer to : https://www.udemy.com/course/the-web-developer-bootcamp
- default parameters, 默认参数放在最后面
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function multiply(a, b = 1) { return a * b; } multiply(3); //3 function multiply(a = 1, b) { return a * b; } multiply(3); //NaN
// ========================================== // AN OLDER WAY OF ADDING DEFAULT PARAM VALUE // ========================================== // function rollDie(numSides) { // if (numSides === undefined) { // numSides = 6 // } // return Math.floor(Math.random() * numSides) + 1 // } // ============ // THE NEW WAY! // ============ function rollDie(numSides = 6) { return Math.floor(Math.random() * numSides) + 1 } function greet(person, msg = "Hey there", punc = '!') { console.log(`${msg}, ${person}${punc}`)
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spread, Spread syntax (
...) allows an iterable such as an array expression or string to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected, or an object expression to be expanded in places where zero or more key-value pairs (for object literals) are expected.- SPREAD for function calls, expands an iterable(array, string, etc.) into a list of arguments
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const nums = [0, 1, 2, 3]; Math.max(nums); //NaN //Use spread! Math.max(...nums); //3 //same as calling: Math.max(0,1,2,3)
- SPREAD in array literals, creat a new array using an existing array. Spreads the elements from one array into a new array.可以
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const nums1 = [1, 2, 3]; const nums2 = [4, 5, 6]; [...nums1, ...nums2]; // [1, 2, 3, 4, 5, 6]
//Append all items from arr2 onto arr1 nums1 = nums1.concat(nums2);
['a', 'b', ...nums2]; // ["a", "b", 4, 5, 6] [...nums1, ...nums2, 7, 8, 9]; //[1,2,3,4,5,6,7,8,9] - SPREAD in object literals, copies properties from one object into another object literal
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const feline = { legs: 4, family: 'Felidae' }; const canine = { isFurry: true, family: 'Caninae' }; const catDog = { ...feline, ...canine }; const dataFromForm = { email: 'blueman@gmail.com', password: 'tobias123!', username: 'tfunke' } const newUser = { ...dataFromForm, id: 2345, isAdmin: false } console.log(catDog); // {legs: 4, family: "Caninae", isFurry: true}, feline和canine都有family这个属性，会被第二个重写 console.log(newUser); // {email: "blueman@gmail.com", password: "tobias123!", username: "tfunke", id: 2345, isAdmin: false}， copy and add.
- The Arguments Object
- Available inside every function
- it is an array-like object
- has a length property
- does not have array methods like push/pop
- contains all the arguments passed to the function
- not available inside of arrow functions!
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function sumAll() { let total = 0; for (let i = 0; i < arguments.length; i++) { total += arguments[i]; } return total; } console.log(sumAll(1, 2, 3, 4, 5)); //15
- Rest , collects all remaining arguments into an actual array. The rest parameter syntax allows a function to accept an indefinite number of arguments as an array, providing a way to represent variadic functions in JavaScript.
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// COLLECT THE "REST" IN NUMS: function sum(...nums) { return nums.reduce((total, el) => total + el) } sum(7, 8, 9, 10); //34, 求和， 也可以
let total = 0;
for(let n of nums) total + = n;
return total; function raceResults(gold, silver, ...everyoneElse) { console.log(`GOLD MEDAL GOES TO: ${gold}`) console.log(`SILVER MEDAL GOES TO: ${silver}`) console.log(`AND THANKS TO EVERYONE ELSE: ${everyoneElse}`) } raceResults("Lily", "Keven", "Bob", "Soffi", "Gorge"); //GOLD MEDAL GOES TO: Lily //SILVER MEDAL GOES TO: Keven //AND THANKS TO EVERYONE ELSE: Bob,Soffi,Gorge
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- Destructuring,a short. clean syntax to "unpack":
- values from arrays
- properties from objects
into distinct variables.
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// =================== // ARRAY DESTRUCTURING // =================== const scores = [929321, 899341, 888336, 772739, 543671, 243567, 111934]; // const highScore = scores[0]; // const secondHighScore = scores[1]; const [gold, silver, bronze, ...everyoneElse] = scores; // gold = 929321, silver = 899341, bronze = 888336, everyoneElse = [772739, 543671, 243567, 111934] // =================== // OBJECT DESTRUCTURING // =================== const user = { email: 'harvey@gmail.com', password: 'sCoTt1948sMiTh', firstName: 'Harvey', lastName: 'Milk', born: 1930, died: 1978, bio: 'Harvey Bernard Milk was an American politician and the first openly gay elected official in the history of California, where he was elected to the San Francisco Board of Supervisors', city: 'San Francisco', state: 'California' } const user2 = { email: 'Stacy@gmail.com', firstName: 'Stacy', lastName: 'Gonzalez', born: 1987, city: 'Tulsa', state: 'Oklahoma' } // const firstName = user.firstName; // const lastName = user.lastName; // const email = user.email; const { email, firstName, lastName, city, bio } = user; //change the name,use "=" to set default value const { born: birthYear, died: deathYear = 'N/A' } = user; const { city, state, died = 'N/A' } = user2; // =================== // PARAM DESTRUCTURING // =================== function fullName(user) { return `${user.firstName} ${user.lastName}` } function fullName(user) { const { firstName, lastName } = user; return `${firstName} ${lastName}` } function fullName({ firstName, lastName }) { return `${firstName} ${lastName}` } const movies = [ { title: 'Amadeus', score: 99, year: 1984 }, { title: 'Sharknado', score: 35, year: 2013 }, { title: '13 Going On 30', score: 70, year: 2004 }, { title: 'Stand By Me', score: 85, year: 1986 }, { title: 'Waterworld', score: 62, year: 1995 }, { title: 'Jingle All The Way', score: 71, year: 1996 }, { title: 'Parasite', score: 95, year: 2019 }, { title: 'Notting Hill', score: 77, year: 1999 }, { title: 'Alien', score: 90, year: 1979 } ] movies.filter((movie) => movie.score >= 90) movies.filter(({ score }) => score >= 90) //use {} to position the score parameter movies.map(movie => { return `${movie.title} (${movie.year}) is rated ${movie.score}` }) movies.map(({ title, score, year }) => { return `${title} (${year}) is rated ${score}` })
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