如何使用构造函数Javascript在嵌套数组中的键值对中输出键

Question

Within the array of 'items' I wanted to output within the 'info' array only the keys: [ 'stringTwo', 'StringThree' ] and also output the value String Three

 let items = [ { string: 'string1', info: { stringTwo:'String Two', stringThree: 'String Three' }, size:3445 }, { string: 'string2', info: 'ruby files' }, { string: 'string3', info: '' }, { string: 'string4 without info key', } ]; 

I tried with both of these codes:

 data.forEach((data) => { if(data.info.constructor === Object) { console.log(Object.keys(data.info)); } }) data.forEach((data) => { if(data.info.constructor === Object) { console.log((data.info.stringThree)); } }) 

the first one should output the Keys [ 'stringTwo', 'StringThree' ] and the second one should output String Three

I'm wondering why in a more larger scale array that has more key-value pairs both doesn't work and gives me an input of TypeError: Cannot read property 'constructor' of undefined ? if so are there other ways without using constructor?

Answer 1

Your larger scale array probably don't have any info key. To prevent having an error, you should :

• Change your items array to put an info key, even empty

  or

• Add a typeof data.info !== "undefined" condition to check if the info key is defined on each item before trying to access it.

Here is a working example :

 let items = [ { string: 'string1', info: { stringTwo:'String Two', stringThree: 'String Three' }, size:3445 }, { string: 'string2', info: 'ruby files' }, { string: 'string3', info: '' }, { string: 'string4 without info key', } ]; items.forEach((data) => { if(typeof data.info !== "undefined" && data.info.constructor === Object) { console.log(data.info.stringThree); } })