The map.keys() function in D3.js is used to return an array of string keys for every entry in the created map. The order of the returned keys is arbitrary.
Syntax:
map.keys()
Parameters: This function does not accept any parameters.
Return Value: This function returns an array of string keys for every entry in the created map.
Below programs illustrate the d3.map.keys() function in D3.js:
Example 1:
<!DOCTYPE html> <html>   <head>     <title>d3.map.keys() function</title>   </head>   <body>     <script>                   // Creating a map         var map = d3.map({"Ram": 5, "Geeks": 10, "gfg": 15});                 // Calling the map.keys() function         A = map.keys();                 // Getting an array of string keys for         // every entry in the map.         console.log(A);     </script> </body>   </html> |
Output:
["Ram", "Geeks", "gfg"]
Example 2:
<!DOCTYPE html> <html>   <head>     <title>d3.map.keys() function</title>   </head>   <body>     <script>                   // Creating some maps         var map1 = d3.map({"Ram": 5});         var map2 = d3.map({"Geeks": 10});         var map3 = d3.map({"Ram": 5, "Geeks": 10});         var map4 = d3.map();                 // Calling the map.keys() function         A = map1.keys();         B = map2.keys();         C = map3.keys();         D = map4.keys();                 // Getting an array of string keys for         // every entry in the map.         console.log(A);         console.log(B);         console.log(C);         console.log(D);     </script> </body>   </html> |
Output:
["Ram"] ["Geeks"] ["Ram", "Geeks"] []
Ref: https://devdocs.io/d3~5/d3-collection#map_keys
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