javascript - Why does Typescript discourage setting values to null? - Stack Overflow

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I don't understand the reasoning behind non nullable types and Typescript's strictNullChecks flag. I always set the value of a variable to null when I am sure I am done with it to release it from memory and the GC can collect it. My tests have shown that it speeds up the execution of the JavaScript code by many times, especially in long, and deep loops. Is this a bad practice? Should some types of variables be set to null and not others?

I don't understand the reasoning behind non nullable types and Typescript's strictNullChecks flag. I always set the value of a variable to null when I am sure I am done with it to release it from memory and the GC can collect it. My tests have shown that it speeds up the execution of the JavaScript code by many times, especially in long, and deep loops. Is this a bad practice? Should some types of variables be set to null and not others?

• javascript
• typescript
• nullable

Share Improve this question Follow asked Sep 28, 2019 at 10:15 scripterscripter 11522 silver badges1212 bronze badges 4

• 1 just out of curiosity, could you share the tests you're talking about? – georg Commented Sep 28, 2019 at 10:17
• The tests I referred to were basically running some code on an older model smart phone and crashing it with an out of memory code, then tweaking the code until it would run again. Consequently the code also ran ~4X faster in a desktop browser (Firefox) as well, probably by avoiding the GC. The code was parsing several MB of CSV text to JSON and storing it in a database. Quite plicated to show here – scripter Commented Sep 28, 2019 at 10:28
• 2 My advice if you're only using null to free up memory (hey, you're the boss): turn on --strictNullChecks, declare your variables as non-nullable (e.g., let x: string = "hello"; and then if you must set a variable to null when you're done with it, lie to the piler about it (e.g., x = null!; where the ! operator is a non-null assertion); safe enough if you're really not using it anymore. Happy to turn this into an answer if it meets your needs. Good luck! – jcalz Commented Sep 29, 2019 at 0:21
• Thanks @jcalz x = null!; fixes all my errors with a simple find and replace while still allowing the strictNullChecks flag to be used. It probably doesn't answer the entire question but was a super tip! – scripter Commented Sep 30, 2019 at 7:15

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2 Answers 2

Sorted by: Reset to default 3

The reasoning is to prevent some run-time errors including console.log(null.printWhyNullIsBad()); by catching those errors at the pilation time while assuming that people who write loops so deep and so nested that it stretches the modern JS garbage collectors to their limits can write that kind of code in C/C++ and run it using WebAssembly.

In most cases you don't have to delete variable values yourself. It may be a design flaw. Also note that if you want to delete an object property value, it may be more consistent to use that:

    delete myObject.attr;  // set 'undefined'

I don't understand the reasoning behind non nullable types and Typescript's strictNullChecks flag.

To understand the logic behind, check this sample:

// strictNullChecks: false

interface Sample {
  color: string;
  width: number;
}

function doSomething(sample: Sample): Sample {
  if(sample.width > 10) {
    return null;
  }
  return sample;
}

const first: Sample = {
  color: 'red',
  width: 15
};

const second: Sample = doSomething(first);

console.log(second.width);  // Uncaught TypeError: Cannot read property 'width' of null

Playground

This code piles only without strictNullChecks flag. In this context when the code runs we have a beautiful error: Uncaught TypeError: Cannot read property 'width' of null.

Check the function doSomething. It returns a Sample type, but without the flag we can return null. The problem here is that when we call this function, the value returned will be manipulated as a Sample => There is a inconsistence between the type and its real value type. It's a huge cause of errors.

I like to think that TS typings must reflect at the best real types of values.

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