React Typescript - variable null check ignored and sends error

Question

Javascript newbie here^^

In my React front, I have the following chrome error message :

Unhandled Rejection (TypeError): Cannot read property 'style' of null

▶ 2 stack frames were collapsed.

new LiquidFillGauge

SOMEPATH/liquidFillGauge.tsx:46

  43 | var gauge = d3.select('#' + elementId);
  44 | var radius = 0, locationX = 0, locationY = 0;
  45 | if (gauge != null && gauge != undefined) {
> 46 |     radius = Math.min(parseInt(gauge.style('width')), parseInt(gauge.style('height'))) / 2;
  47 | ^   locationX = parseInt(gauge.style('width')) / 2 - radius;
  48 |     locationY = parseInt(gauge.style('height')) / 2 - radius;
  49 | }

gauge being exceedingly checked in line 45, it still appears null in line 46.

Why is that happening ? How could I avoid the error ?

Thank you

angelo · Accepted Answer · 2020-12-31 10:52:15Z

1

Just do

if(gauge){
   radius = Math.min(parseInt(gauge.style('width')), parseInt(gauge.style('height'))) / 2; 
   locationX = parseInt(gauge.style('width')) / 2 - radius;
   locationY = parseInt(gauge.style('height')) / 2 - radius;
}

In javascript or typescript a null or undefined returns false.

answered Dec 31, 2020 at 10:52

angelo

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Comments

Constantin · Accepted Answer · 2020-12-31 10:52:23Z

1

Because == stands just for equal type. You ought to use === which stands for equal type and equal value.

See this example:

let a = null;
let isNull = a != null;

isNull = false

To check null, undefined, false, you could do:

isEmpty = !gauge;

answered Dec 31, 2020 at 10:52

Constantin

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