我的 for 循环无法正常工作，但我不知道为什么

Question

So basically I'm writing a script that takes in a credit card number and outputs the provider name. (Also if im doing this in an overly complicated way advice is appreciated)

But my initial for loop to iterate over each object is not working. It will get to 3 (even though the length of the arr is 8) and then just stop...

I am dumbfounded as I have re-read the code for an hour now, checked everything I could think of but am just hitting a wall. Any help would be much appreciated!

The Code

var detectNetwork = function(cardNumber) {
var splitDigits = cardNumber.split('');
var length = splitDigits.length;
var first = splitDigits[0];
var second = splitDigits[1];
var firstTwo = splitDigits[0] + splitDigits[1];
var firstThree = splitDigits[0] + splitDigits[1] + splitDigits[2];
var firstFour = splitDigits[0] + splitDigits[1] + splitDigits[2] + splitDigits[3];
var firstSix = splitDigits[0] + splitDigits[1] + splitDigits[2] + splitDigits[3] + splitDigits[4] + splitDigits[5];

var checkProvider = [
    {
        name: 'American Express',
        length: [ 15 ],
        prefixLength: [ 2 ],
        prefixType: 'simple',
        prefix: [ 34, 37 ]
    },

    {
        name: 'Diner\'s Club',
        length: [ 14 ],
        prefixLength: [ 2 ],
        prefixType: 'simple',
        prefix: [ 38, 39 ]
    },

    {
        name: 'Visa',
        length: [ 13, 16, 19 ],
        prefixLength: [ 1 ],
        prefixType: 'simple',
        prefix: [ 4 ]
    },

    {
        name: 'MasterCard',
        length: [ 16 ],
        prefixLength: [ 2 ],
        prefixType: 'range',
        prefix: [ 51, 55 ]
    },

    {
        name: 'Discover',
        length: [ 16, 19 ],
        prefixLength: [ 2, 3, 4],
        prefixType: 'complexRange',
        prefix: [ 6011, [ 644, 649 ], 65 ]
    },

    {
        name: 'Maestro',
        length: [ 12, 13, 14, 15, 16, 17, 18, 19],
        prefixLength: [ 4 ],
        prefixType: 'simple',
        prefix: [ 5018, 5020, 5038, 6304 ]
    },

    {
        name: 'China UnionPay',
        length: [ 16, 17, 18, 19 ],
        prefixLength: [ 3, 4, 6 ],
        prefixType: 'complexRange',
        prefix: [ [ 622126, 622925 ], [ 624, 626 ], [ 6282, 6288 ] ]
    },

    {
        name: 'Switch',
        length: [ 16, 18, 19 ],
        prefixLength: [ 4, 6 ],
        prefixType: 'simple',
        prefix: [ 4903, 4905, 4911, 4936, 564182, 633110, 6333, 6759 ]
    }
];

for(var i = 0; i < checkProvider.length; i++) {
    var currentProvider = checkProvider[i];
    var name = currentProvider.name;
    var lengthOptions = currentProvider.length;
    var prefixLengthOptions = currentProvider.prefixLength;
    var prefixType = currentProvider.prefixType;
    var prefixOptions = currentProvider.prefix;

    for(var j = 0; j < lengthOptions.length; j++) {
        var currentLength = lengthOptions[j];
        if (currentLength === length) {
            //Use first
            for (var y = 0; y < prefixLengthOptions.length; y++) {
                var currentPrefixLength = prefixLengthOptions[y];
                if(currentPrefixLength === 1) {
                    if (first === '4') {
                        if (firstFour !== 4903 && firstFour !== 4905 && firstFour !== 4911 && firstFour !== 4936) {
                            return name;
                        }
                    }
                }
                if (currentPrefixLength === 2) {
                    if (prefixType === 'complexRange') {
                        console.log(name);
                    }
                    if (prefixType === 'simple') {
                        for (var i = 0; i < prefixOptions.length; i++) {
                            if (firstTwo === prefixOptions[i].toString()) {
                                return name;
                            }
                        }
                    }
                    if (prefixType === 'range') {
                        var min = prefixOptions[0];
                        var max = prefixOptions[1];
                        for (var i = min; i < max + 1; i++) {
                            if (firstTwo === i.toString()) {
                                return name;
                            }
                        }
                    }  
                }
            }
        }
    }
}
}

Answer 1

Your test detectNetwork('6011123456789303') logs Discover if you make the following small changes to your code:

1. Replace all var with const .
2. Declare all loop index variables with let . For example, for (let i = 0; .

Answer 2

The issue with your current code is that you have var i redeclared within the same context. According to documentation , these cases do not trigger an error, however the behavior will be confusing when the values are overwritten, which is your case.

The suggestion is to rename the other var i to something else, and that should solve your problem.

Another option is to use let i which will make the context more strict .

So this code would work best (I made it so if always returns the name):

const detectNetwork = function(cardNumber) {
const splitDigits = cardNumber.split('');
const length = splitDigits.length;
const first = splitDigits[0];
const second = splitDigits[1];
const firstTwo = splitDigits[0] + splitDigits[1];
const firstThree = splitDigits[0] + splitDigits[1] + splitDigits[2];
const firstFour = splitDigits[0] + splitDigits[1] + splitDigits[2] + splitDigits[3];
const firstSix = splitDigits[0] + splitDigits[1] + splitDigits[2] + splitDigits[3] + splitDigits[4] + splitDigits[5];

const checkProvider = [
    {
        name: 'American Express',
        length: [ 15 ],
        prefixLength: [ 2 ],
        prefixType: 'simple',
        prefix: [ 34, 37 ]
    },

    {
        name: 'Diner\'s Club',
        length: [ 14 ],
        prefixLength: [ 2 ],
        prefixType: 'simple',
        prefix: [ 38, 39 ]
    },

    {
        name: 'Visa',
        length: [ 13, 16, 19 ],
        prefixLength: [ 1 ],
        prefixType: 'simple',
        prefix: [ 4 ]
    },

    {
        name: 'MasterCard',
        length: [ 16 ],
        prefixLength: [ 2 ],
        prefixType: 'range',
        prefix: [ 51, 55 ]
    },

    {
        name: 'Discover',
        length: [ 16, 19 ],
        prefixLength: [ 2, 3, 4],
        prefixType: 'complexRange',
        prefix: [ 6011, [ 644, 649 ], 65 ]
    },

    {
        name: 'Maestro',
        length: [ 12, 13, 14, 15, 16, 17, 18, 19],
        prefixLength: [ 4 ],
        prefixType: 'simple',
        prefix: [ 5018, 5020, 5038, 6304 ]
    },

    {
        name: 'China UnionPay',
        length: [ 16, 17, 18, 19 ],
        prefixLength: [ 3, 4, 6 ],
        prefixType: 'complexRange',
        prefix: [ [ 622126, 622925 ], [ 624, 626 ], [ 6282, 6288 ] ]
    },

    {
        name: 'Switch',
        length: [ 16, 18, 19 ],
        prefixLength: [ 4, 6 ],
        prefixType: 'simple',
        prefix: [ 4903, 4905, 4911, 4936, 564182, 633110, 6333, 6759 ]
    }
];

for(let i = 0; i < checkProvider.length; i++) {
    let currentProvider = checkProvider[i];
    let name = currentProvider.name;
    let lengthOptions = currentProvider.length;
    let prefixLengthOptions = currentProvider.prefixLength;
    let prefixType = currentProvider.prefixType;
    let prefixOptions = currentProvider.prefix;

    for(let j = 0; j < lengthOptions.length; j++) {
        let currentLength = lengthOptions[j];
        if (currentLength === length) {
            //Use first
            for (let y = 0; y < prefixLengthOptions.length; y++) {
                let currentPrefixLength = prefixLengthOptions[y];
                if(currentPrefixLength === 1) {
                    if (first === '4') {
                        if (firstFour !== 4903 && firstFour !== 4905 && firstFour !== 4911 && firstFour !== 4936) {
                            return name;
                        }
                    }
                }
                if (currentPrefixLength === 2) {
                    if (prefixType === 'complexRange') {
                        return(name);
                    }
                    if (prefixType === 'simple') {
                        for (let i = 0; i < prefixOptions.length; i++) {
                            if (firstTwo === prefixOptions[i].toString()) {
                                return name;
                            }
                        }
                    }
                    if (prefixType === 'range') {
                        let min = prefixOptions[0];
                        let max = prefixOptions[1];
                        for (let i = min; i < max + 1; i++) {
                            if (firstTwo === i.toString()) {
                                return name;
                            }
                        }
                    }  
                }
            }
        }
    }
}
}

var n = detectNetwork('6011123456789303');
console.log(n);  // Discover