Published Oct 13, 2019
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New function for assigning enumerable properties of one or more source objects onto a destination object.
var dest = { quux: 0 }
var src1 = { foo: 1, bar: 2 }
var src2 = { foo: 3, baz: 4 }
Object.assign(dest, src1, src2)
dest.quux === 0
dest.foo === 3
dest.bar === 2
dest.baz === 4
New function for finding an element in an array.
[ 1, 3, 4, 2 ].find(x => x > 3) // 4
[ 1, 3, 4, 2 ].findIndex(x => x > 3) // 2
New string repeating functionality.
" ".repeat(4 * depth)
"foo".repeat(3)
New specific string functions to search for a sub-string
"hello".startsWith("ello", 1) // true
"hello".endsWith("hell", 4) // true
"hello".includes("ell") // true
"hello".includes("ell", 1) // true
"hello".includes("ell", 2) // false
New functions for checking for non-numbers and finite numbers
Number.isNaN(42) === false
Number.isNaN(NaN) === true
Number.isFinite(Infinity) === false
Number.isFinite(-Infinity) === false
Number.isFinite(NaN) === false
Number.isFinite(123) === true
Checking whether an integer number is in the safe range, i.e., it is correctly represented by JavaScript (where all numbers, including integer numbers, are technically floating point number).
Number.isSafeInteger(42) === true
Number.isSafeInteger(9007199254740992) === false
Availability of a standard Epsilon value for more precise comparison of floating point numbers.
console.log(0.1 + 0.2 === 0.3) // false
console.log(Math.abs((0.1 + 0.2) - 0.3) < Number.EPSILON) // true
Truncate a floating point number to its integral part, completely dropping the fractional part.
console.log(Math.trunc(42.7)) // 42
console.log(Math.trunc( 0.1)) // 0
console.log(Math.trunc(-0.1)) // -0
Determine the sign of a number, including special cases of signed zero and non-number.
console.log(Math.sign(7)) // 1
console.log(Math.sign(0)) // 0
console.log(Math.sign(-0)) // -0
console.log(Math.sign(-7)) // -1
console.log(Math.sign(NaN)) // NaN
First class representation of value that may be made asynchronously and be available in the future.
function msgAfterTimeout (msg, who, timeout) {
return new Promise((resolve, reject) => {
setTimeout(() => resolve(`${msg} Hello ${who}!`), timeout)
})
}
msgAfterTimeout("", "Foo", 100).then((msg) =>
msgAfterTimeout(msg, "Bar", 200)
).then((msg) => {
console.log(`done after 300ms:${msg}`)
})
Combine one or more promises into new promises without having to take care of ordering of the underlying asynchronous operations yourself.
function fetchAsync (url, timeout, onData, onError) {
…
}
let fetchPromised = (url, timeout) => {
return new Promise((resolve, reject) => {
fetchAsync(url, timeout, resolve, reject)
})
}
Promise.all([
fetchPromised("https://backend/foo.txt", 500),
fetchPromised("https://backend/bar.txt", 500),
fetchPromised("https://backend/baz.txt", 500)
]).then((data) => {
let [ foo, bar, baz ] = data
console.log(`success: foo=${foo} bar=${bar} baz=${baz}`)
}, (err) => {
console.log(`error: ${err}`)
})
References: