[Typescript] Make Typescript Stick

Refer: https://www.cnblogs.com/Answer1215/p/15084496.html

A string is a primitive value, and all primitive values are immutable.

Q1: 

const a = "Frontend Masters"
let b = "Frontend Masters"
 
const c = { learnAt: "Frontend Masters" }
let d = { learnAt: "Frontend Masters" }
 
const e = Object.freeze({ learnAt: "Frontend Masters" })

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Answer:

All primitive values are immutable, therefore a & b are both hold immutable values.

Repeat "All primitive values are immutable" three times...

All primitive values are immutable

All primitive values are immutable

All primitive values are immutable

 

Const and Let differ in terms of whether variables can be reassigned, but that has nothing to do with whether the values they hold can be modified.

Therefore both c & d does NOT hod ummutable values

 

Object.freeze prevents properties of an object from being changed, and prevents new properties from being added. This effectively is a "Shallow immutability". Which means Object.freeze won't help you mutate a nested object.

const e = Object.freeze({ course: 'Typescript', learnAt: {place: "Frontend Masters"} })
e.course = "Reacct" // report error
e.learnAt.place = "Udmey" // works

 

Q2:

const str = "hello"
let val =
  /* ??? */
  console.log(val)
/**
 * {
 *   '0': 'h',
 *   '1': 'e',
 *   '2': 'l',
 *   '3': 'l',
 *   '4': 'o'
 * }
 */

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Answer:

let val = { ...str.split("") }
console.log(val)
/**
 * {
 *   '0': 'h',
 *   '1': 'e',
 *   '2': 'l',
 *   '3': 'l',
 *   '4': 'o'
 * }
 */

 

Q3:

Look at the types of first and second below, as well as the compile error messages. What does your mental model tell you about how string and String are different?

let first: string & number let second: String & Number   first = "abc" Type 'string' is not assignable to type 'never'. second = "abc" Type 'string' is not assignable to type 'String & Number'. Type 'string' is not assignable to type 'Number'. second = new String("abc") Type 'String' is not assignable to type 'String & Number'. Type 'String' is missing the following properties from type 'Number': toFixed, toExponential, toPrecision

for the first: both string and number are primtive types, they are not such a value is both string and number at the same time, so that it results in a `never`.

for the second: String and Number are interface types. The Union of String and Number are common methods and props of both, but it won't results in a never type.

 

Q4:

In what order will the animal names below be printed to the console?

function getData() {
  console.log("elephant")
  const p = new Promise((resolve) => {
    console.log("giraffe")
    resolve("lion")
    console.log("zebra")
  })
  console.log("koala")
  return p
}
async function main() {
  console.log("cat")
  const result = await getData()
  console.log(result)
}
console.log("dog")
main().then(() => {
  console.log("moose")
})

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Answer:

"dog" will come first

Then main() function will be invoked, so "cat" will come second.

Then getData() function will be invoked, so "elephant" will come third.

new Promise() constructor function will be invoked, 4th. "giraffe"

resolve() function will be invoked by the time we call .then()

so "zebra" comes 5th.

then "koala" comes 6th.

after await getData(), console.log(result) will run, so "lion" 7th.

"moose" will be last

• Are you surprised that giraffe and zebra happen so early? Remember that Promise executors are invoked synchronously in the Promise constructor
• Are you surprised that lion happens so late? Remember that a resolve is not a return. Just because a Promise has resolved, doesn’t mean the corresponding .then (or await is called immediately)