TypeError: Cannot read properties of undefined (reading 'id')

Question

I have this error in my terminal:

TypeError: Cannot read properties of undefined (reading 'id')

I'm trying to test the call to an API, but the error appears.

My function:

itemToForm = () => {
    this.api.send(this.component, 'get',
        { lang: 'ES', filter: { id: this.item['id'] } }
    ).then(resEsp => {
        this.item = resEsp['data'][0];
        this.api.send(this.component, 'get',
            { lang: 'EN', filter: { id: this.item['id'] } }
        ).then(res => {
            let itemEng = res['data'][0];
            let fields = this.formDef.map(register => register.filter(
                field => field['register_table'].indexOf('traduction') !== -1
            ).map(
                field => field['field_name'])
            ).filter(register => register.length);

            fields = fields.length ? fields[0] : [];

            if (itemEng) {
                this.item = Object.keys(itemEng).reduce((obj, key) => {
                    obj[key] = this.item[key];
                    if (fields.indexOf(key) !== -1) {
                        obj[key + '_eng'] = itemEng[key];
                    }
                    return obj;
                }, {});
            }

            if (this.item) {
                this.setForm();
            }
        })
    })
}

My specification file:

it('should call api.send', () => {
    let spy1 = spyOn(api, 'send');
    let item = {
        id: 1,
        name: 'test',
    }

    component.addItem(item);
    component.itemToForm();

    expect(spy1).toHaveBeenCalled();
});

Answer 1

What is happening:

The function itemToForm() is being called before the this.item is ready.

There are many strategies to avoid this error. A very simple one is to add a catcher at the beginning of the function, like this:

itemToForm = () => {
  if(this.item === undefined) {return}
         
  // The rest of the code
}

This stops the function, if the data does not exist yet.

A more elegant solution may be to go further up in your order of operations, and find who is calling itemToForm() and ensure the data exists prior to calling.

Answer 2

I bumped to this question but my issue was actually something completely different.

In my code for some reasons I had

import { SOME_OBJECT } from '.';

which should instead be like this:

import { SOME_OBJECT } from './proper-file';

Answer 3

In case it's useful for anyone to know, I got the error Cannot read properties of undefined (reading 'spec') when attempting to install a local npm module with npm install [path to module here] . The source of the problem: I hadn't given the local module a name in its package.json .

Answer 4

在我的情况下，错误是相似的，但我得到了不同的解决方案，因为我在 JS 文件中使用 body-parser 模块，我必须添加

app.use(bodyParser.urlencoded({ extended: true }));

Answer 5

Typescript version error in package.json change typescript version to,

"typescript": "3.8.3"

then npm install