JSFiddle - React, Tailwind, and code Playground

JavaScript

// "Simple" solution: use an array to hold the keys:
var aCities = [];
var sCity = "Vancouver";
aCities[sCity] = true;

// ... omit: more code to add more cities dynamically to the list...

alert(aCities.indexOf("Burnaby") >= 0); // Somewhat ugly, need to test numeric index instead of nice boolean.

// Now if we want to remove a value, we need to find the index, then use a splice() call, etc. and it gets very complicated.

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// Instead, use an Object as a map to hold the keys:
var oCities = {};
var sCity = "Vancouver";
oCities[sCity] = true;

// ... omit: more code to add more cities dynamically to the list...

// Now we can easily test whether a particular city is in the list:
var bInList = oCities.hasOwnProperty(sCity);
alert(bInList);

// And we can easily remove an element from the list:
delete oCities[sCity];
var bInList = oCities.hasOwnProperty(sCity);
alert(bInList);