如何从JavaScript数组中删除项目？

Question

I am starting to learn coding with JavaScript and our teacher told us to search this site. There is no answer for my stuff so I wanted to ask. I hope that is OK, I have searched a lot of questions already but I can't find anything that is like my question.

My task is to go through my timetable and take out my two least favourite subject. This is what I have:

var subjects = [
  "Maths", "History", "English", "Science", "Spanish", 
  "Physical Education", "History", "English", "Science", 
  "Maths", "History", "English", "Spanish", "Physical Education"
];

I said I wanted to take out Spanish and History and I did that:

for (var i = 0; i < 14; i++) {
   if (subjects[i] == "Spanish") {
   delete subjects[i];
  }
  if (subjects[i] == "History") {
    delete subjects[i];
  }
}

But this says it has "empty slots" :

Array [ "Maths", <1 empty slot>, "English", "Science", <1 empty slot>, "Physical Education", <1 empty slot>, "English", "Science", "Maths", 4 more… ]

But it should simply not be in there anymore. How can I do that?

Answer 1

As you've found out, arrays can be "sparse" (not all indexes must have a value) and that's what you've accomplished with delete . You deleted the data, but left the index.

To remove the index completely, use the .splice() method for this:

 var subjects = [ "Maths", "History", "English", "Science", "Spanish", "Physical Education", "History", "English", "Science", "Maths", "History", "English", "Spanish", "Physical Education" ]; for (var i = 0; i < 14; i++) { if (subjects[i] == "Spanish") { subjects.splice(i, 1); // At the current index, remove one element } if (subjects[i] == "History") { subjects.splice(i, 1); // At the current index, remove one element } } console.log(subjects); 

Answer 2

Use splice :

 var subjects = ["Maths", "History", "English", "Science", "Spanish", "Physical Education", "History", "English", "Science", "Maths", "History", "English", "Spanish", "Physical Education"]; function remove(arr, item) { for(var i = arr.length; i--;) { if(arr[i] === item) { arr.splice(i, 1); } } } remove(subjects, "Spanish"); remove(subjects, "History"); document.write(subjects); 

Output:

[ 'Maths',
  'English',
  'Science',
  'Physical Education',
  'English',
  'Science',
  'Maths',
  'English',
  'Physical Education' ]

Answer 3

Try it:

for (var index in subjects){
    if (subjects[index] == "Spanish"){
        subjects.splice(index,1);

    }
    if (subjects[index] == "History"){
        subjects.splice(index,1);

    } 
}

Answer 4

var index = array.indexOf('history');
if (index > -1) {
    array.splice(index, 1);
}

The second parameter of splice is the number of elements to remove. Note that splice modifies the array in place and returns a new array containing the elements that have been removed.

Answer 5

As you figured out the delete operator just deletes the item at a location in an array. This spot is still there, but what was in that space is just empty now (arrays can have empty spaces). See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/delete#Deleting_array_elements

You can use splice (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/splice ) to remove items by index position.

 var subjects = [ "Maths", "History", "English", "Science", "Spanish", "Physical Education", "History", "English", "Science", "Maths", "History", "English", "Spanish", "Physical Education" ]; for (var i = 0; i < subjects.length; i++) { if (subjects[i] === "Spanish") { subjects.splice(i, 1); } else if (subjects[i] === "History") { subjects.splice(i, 1); } } console.log(subjects); 

Or you can use filter (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter ):

 var subjects = [ "Maths", "History", "English", "Science", "Spanish", "Physical Education", "History", "English", "Science", "Maths", "History", "English", "Spanish", "Physical Education" ]; var isSubjectILike = function(subject){ return subject !== "Spanish" && subject !== "History"; }; subjects = subjects.filter(isSubjectILike); console.log(subjects); 

Answer 6

You could do it like this:

var subjects = [
  "Maths", "History", "English", "Science", "Spanish", 
  "Physical Education", "History", "English", "Science", 
  "Maths", "History", "English", "Spanish", "Physical Education"
];
var index = subjects.indexOf("Spanish");
subjects = Object.assign(subjects.slice(0, index), subjects.slice(index + 1, subjects.length -1);

Or by using ES6 filter:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter

var subjects = [
  "Maths", "History", "English", "Science", "Spanish", 
  "Physical Education", "History", "English", "Science", 
  "Maths", "History", "English", "Spanish", "Physical Education"
];
subjects = subjects.filter((value)=> value!="Spanish");