在 function 中返回多个值

Question

I'm practicing using the map method and i'm stuck on this problem. I'm trying to return the largest number out of each of these arrays but for my function to be complete i need to return the push method outside of my nested map. I have no idea how to do that. Any input would be extremely appreciated. Thanks in advance.

 const bigAssArr = [[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [100, 1001, 857, 1]] const biggestFour = []; const largestOfFour = (arr) => { return arr.map((item, index) => { let tempMax = item[0]; return item.map((i, iTwo) => { let currentValue = i; if (currentValue > tempMax ) { tempMax = currentValue } }) biggestFour.push(tempMax); // *** This is where i'm having the problem*** }) }

Answer 1

In ES6, you can use Math.max(...arr) to get the max number of an array.

The Math.max() function returns the largest of the zero or more numbers given as input parameters.

For example:

 var arr = [4, 5, 1, 3];
 console.log(Math.max(...arr )); // Output: 5

After that, you can use .map to create a new array.

 const bigAssArr = [[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [100, 1001, 857, 1]] const largestOfFour = (arr) => { return arr.map(items => Math.max(...items)) } console.log(largestOfFour(bigAssArr));

⚠️ According to @GBra's recommendation, Math.max(...arr) is not an optimal solution. It's slow, if the array contains too many elements it will fail or return the wrong result or hit max call stack size.

[Highly recommend !!!] Another option is to use .reduce to get the max item Of an array

 const bigAssArr = [[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [100, 1001, 857, 1]] const getMaxItemOfArray = (arr) => arr.reduce((result, currentItem) => { return Math.max(currentItem, result); }, arr[0]); const result = bigAssArr.map(r => getMaxItemOfArray(r)); console.log(result);

Answer 2

You can find the largest in each array as follow.

 const bigAssArr = [[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [100, 1001, 857, 1]] var biggestFour = bigAssArr.map( arr => { return Math.max(...arr); }) console.log(biggestFour)

Answer 3

 const myArr = [[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [100, 1001, 857, 1]] let highest=arr=>arr.map(a=>Math.max(...a)) const highestValues=highest(myArr) console.log(highestValues)

Answer 4

Use map when you want to return a value for each element of an array

Use reduce when you want to return a single value based of the array, reduce will run the callback function for each element of the array, just like map

reduce takes three arguments:

1. accumulator(this defaults to the first element in the array if no third argument is supplied, in the end reduce will return this value)
2. current value(this defaults to the second element in the array if no third argument is supplied, otherwise this starts from the first element)
3. starting accumulator value

If there is only one element in the array, reduce will have no effect.

 const bigAssArr = [[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [100, 1001, 857, 1]] const largestOfFour = (arr) => { return arr.map((item) => { return item.reduce((acc, curr) => { if (acc > curr) { return acc; } return curr; }) }) } console.log(...largestOfFour(bigAssArr))