数据为空时，Java代码会中断

Question

I am using the following code which works great but totally stops working when "thsub" is null and does not continue reading the rest of the data and just returns a TypeError saying "thsub in null"

Here is the code:

var data = {
  "cars": [{
      "id": "1",
      "name": "name 1",
      "thsub": [{
        "id": "11",
        "name": "sub 1",
        "stats": {
          "items": 5,
        },
        "ions": null
      }, {
        "id": "22",
        "name": "sub 2",
        "stats": {
          "items": 5,
        },
        "translations": null
      }],
      "image": null
    },

    {
      "id": "2",
      "name": "name 2",
      "thsub": null, //this will break the code
      "image": null
    }
  ]
}



var thCount = [];

for (var l = 0, m = data.cars.length; l < m; l++) {
  thCount[l] = 0;
  for (var i = 0, j = data.cars[l].thsub.length; i < j; i++) {
    if (data.cars[l].thsub[i].stats) {
      thCount[l]+=data.cars[l].thsub[i].stats.items;
    }
  }
}

console.log(thCount);

How can I fix this?

Answer 1

It breaks because you're referencing thsub 's length property. As null has no properties, and cannot, this will throw an error. This can, in this situation and every other situation where you're using null, easily be worked around by adding some sort of conditional that will either break your loop or avoid executing the code.

Here's a simple example:

for (var l = 0, m = data.cars.length; l < m; l++) {
  if (data.cars[l].thsub) {
    for (var i = 0, j = data.cars[l].thsub.length; i < j; i++) {
      if (data.cars[l].thsub[i].stats) {
        thCount[l]+=data.cars[l].thsub[i].stats.items;
      }
    }
  }
}

Note the added data.cars[l].thsub -- if this value is null, the condition will evaluate to false, and the exception-causing code will never execute.

Answer 2

Just add a guard condition before you access a property as, an error will be thrown if you try to invoke properties on null or undefined objects.

for (var l = 0, m = data.cars.length; l < m; l++) {
  thCount[l] = 0;
  let tsubs = data.cars[l].thsub || []; <--- Guard condition
  for (var i = 0, j = tsubs.length; i < j; i++) {
    if (data.cars[l].thsub[i].stats) {
      thCount[l] += data.cars[l].thsub[i].stats.items;
    }
  }
}

In the fixed use case tsubs will be set to an empty array when ever the property access returns a falsy value.

Answer 3

Your loop is checking the length attribute of thsub. If thsub is null, then your code is attempting to check an attribute (length) of a null object, which will always break your code.. Try adding a check for the thsub object itself before checking the length:

for (var l = 0, m = data.cars.length; l < m; l++) {
  thCount[l] = 0;
  if (data.cars[l].thsub) {
    for (var i = 0, j = data.cars[l].thsub.length; i < j; i++) {
      if (data.cars[l].thsub[i].stats) {
        thCount[l]+=data.cars[l].thsub[i].stats.items;
      }
    }
  }
}

Working sample:

https://jsfiddle.net/mspinks/bwmwntay/5/

Answer 4

Just check the thsub variable first:

var thsub;

for (var l = 0, m = data.cars.length; l < m; l++) {
  thCount[l] = 0;
  thsub = data.cars[l].thsub;

  if (!(thsub instanceof Array)) continue; // If it's not an array, skip it.

  for (var i = 0, j = thsub.length; i < j; i++) {
    if (!thsub[i].stats) continue; // If it doesn't contain 'stats', skip it.

    thCount[l] += thsub[i].stats.items;
  }
}