When pushing objects to array, how to not push same object?

Question

I'm creating new array with objects checked in the checkbox. But when i submit again, there's a error "flattenChildren(...): Encountered two children with the same key, .$3 . Child keys must be unique; when two children share a key, only the first child will be used." I want push only unique objects.

I`m using react

handleSubmit(){

    let students = []
    for(let idx in this.state.checked){
        if(this.state.checked[idx] ){
            students.push(this.state.allStudent[idx])
        }
    }
    console.log('students', students)
    this.setState({
        studentList: update(this.state.studentList, {$push: students})
    })

}

Answer 1

As you are using React, you should be able to safely use the ES6 Set Object, which lets you store unique values of any type. ( https://devdocs.io/javascript/global_objects/set ).

eg

handleSubmit(){

let students = []
for(let idx in this.state.checked){
    if(this.state.checked[idx] ){
        students.push(this.state.allStudent[idx])
    }
}

students = [...new Set(students)];

console.log('students', students)
this.setState({
    studentList: update(this.state.studentList, {$push: students})
})

}

Answer 2

I am not an expert in React.js, but this seems to be a simple problem(pushing only unique elements to a JS array). Your function modified as follows should work:

handleSubmit(){
    let students = [...this.state.studentList];
    for(let idx in this.state.checked){
        if(this.state.checked[idx] && (students.indexOf(this.state.checked[idx]) === -1)){
            students.push(this.state.allStudent[idx])
        }
    }
    console.log('students', students)
    this.setState({
        studentList: update(this.state.studentList, students)
    })
}

What we have essentially done here is use the 'indexOf()' method on the 'students' array to check if the current element exists in the array - the indexOf() method returns -1 if an element is not found in an array. Hope this helps!