preface

Cut the crap and get to the point.

1 forEach (make each item in the array do one thing)

If you don’t do something special, in general, it won’t break. If you need to break out of the loop, use the for statement instead.

let arr = [1.2.3.4.5];
arr.forEach((item,index) = > {
    console.log(item);
})
/ / 1, 2, 3, 4, 5
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2 map (make an array produce a new array by some calculation)
let arr = [1.2.3.4.5];
let newArray = arr.map((item,index) = > {
    return item * 2
});
newArray  // [2, 4, 6, 8, 10]
arr / / [1, 2, 3, 4, 5]
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This returns a new array, it doesn’t change the original array.

3 reduce (Do some sort of calculation on the front and back of the array and add up the final value)
let arr = [1.2.3.4.5];
let result = arr.reduce((prev, cur, index, arr) = > {
    return prev + cur;
});
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CallbackFunction contains four parameters, to explain what they mean:

Prev: the value of the first item or the result of the last stack cur: the current item that will be added index: the current index arr: the array itself

In this case, the default initialValue is [0] of the array. If you want to customize the initialValue, you can add a second parameter (initialValue) to reduce, as follows

let result = arr.reduce((prev, cur, index, arr) = > {
    return prev + cur;
}, - 5);
// If the initialValue is a sum, be sure to note the type of the initialValue. If set to a string, the result is a string according to "add them together and the value type is the same as the first addend".
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4 filter (Returns a new array if the item in the filter array matches the criteria)
let arr = [1.2.3.4.5];
let newArray = arr.filter((item,index) = > {
    return item > 3;
});
 newArray / / (4, 5)
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5 every (checks whether each item in the array meets the criteria)

You can think of it as ampersand and

let arr = [1.2.3.4.5];
let result = arr.every((item,index) = > {
    return item > 0;
});
 result  // true
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6 some (Check if any items in the array match the criteria)

Can understand as | | ‘or’ relationship

let arr = [1.2.3.4.5];
let result = arr.some((item,index) = > {
    return item > 2;
});
 result  // true
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