How to check whether a property is present in ReactNative?

Question

In ReactNative, I have a property that might or might not be passed down to a class object. Is there a way like in Java to check to see the property is null or not?

In my case, I have this.props.overrideAccessibilityLabel that might be passed down or not. I only want to use it when it is passed down:

  render() {
    return (
      <View 
        accessibilityLabel={this.props.currMessage.text}
        if {...props.myAccessibilityLabel} {
          ...accessibilityLabel={...props.myAccessibilityLabel}
        }
        >
        <Text1
           // ...
        >
        </Text1>
      </View>
    );
  }

Do you have any code? A specific instance that this is useful? Otherwise, this is a duplicate of how-do-i-check-if-an-object-has-a-property-in-javascript — Skam
– Skam, Commented Jul 29, 2017 at 22:11

Liwen Zhao · Accepted Answer · 2017-08-06 19:47:41Z

4

But then a one-liner "if" statement works the best in this case.

      <View 
        accessibilityLabel={this.props.currMessage.text ?
this.props.currMessage.text : this. props.myAccessibilityLabel}
        >
        <Text1
           // ...
        >
        </Text1>
      </View>

edited Aug 6, 2017 at 19:47

answered Aug 6, 2017 at 17:51

Liwen Zhao

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Huifang Feng · Accepted Answer · 2017-08-06 02:42:17Z

3

In ReactNative, when a property is not present, it is evaluated to false. (An empty string is also evaluated to false.) So using "if" to check whether a property is present is the right way to do.

function myFunc(x) {
    if (x) {
        return true;
    } else {
        return false;
    }
}
var a1 = {}
a1.a = "abc"
a1.b = ""
var ra=myFunc(a1.a)
var rb=myFunc(a1.b)
var rc=myFunc(a1.b)
console.log("ra=" + ra + "; rb=" + rb + "; rc=" + rc)

The output is:

ra=true; rb=false; rc=false

So a property is treated differently from a var. Checking an empty property just results in false, whereby referencing a undefined var causes an exception.

edited Aug 6, 2017 at 2:42

answered Aug 6, 2017 at 2:31

Huifang Feng

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