The case of disappearing this pointer | Hashnode

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Reza Razavipour

node.js developer

Feb 7, 2017

The case of disappearing this pointer

I have a class with a member function and a member variable. In the member function, I display the this pointer and it is valid. In the same routine, inside a foreach loop, the this pointer becomes undefined

The code is

public getTypes(vitalsModel) : Array<any>{
    var vitals = _.get(vitalsModel, 'data');
    if (_.isEmpty(vitals) || !_.isArray(vitals)) {
        return [];
    }

   // the this pointer is valid
    _.forEach(vitals, function (vital) {
            let setting = _.get(vitalsModel, key);

          // here the this pointer is undefined.
            this.generalSettings.push(setting);

        console.log(keys);
        rows.push(keys);
        loc++;
    });

Anyone sees what i am doing wrong?

#javascript #typescript

Responses(1)

As a fix, you could assign this to another variable, say self, and use that instead.

This should work:

public getTypes(vitalsModel) : Array<any>{
    var vitals = _.get(vitalsModel, 'data');
    if (_.isEmpty(vitals) || !_.isArray(vitals)) {
        return [];
    }

    // assign the 'this' context to a variable
    var self = this;
    _.forEach(vitals, function (vital) {
        let setting = _.get(vitalsModel, key);

        // 'this' has a different context here
        self.generalSettings.push(setting);

        console.log(keys);
        rows.push(keys);
        loc++;
    });
    ...

You could read this article to get a good understanding of how this works in JavaScript

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The case of disappearing this pointer

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