1+ /*
2+
3+ Given a function fn, return a new function that is identical to the original function except that it ensures fn is called at most once.
4+
5+ The first time the returned function is called, it should return the same result as fn.
6+ Every subsequent time it is called, it should return undefined.
7+
8+
9+ Example 1:
10+
11+ Input: fn = (a,b,c) => (a + b + c), calls = [[1,2,3],[2,3,6]]
12+ Output: [{"calls":1,"value":6}]
13+ Explanation:
14+ const onceFn = once(fn);
15+ onceFn(1, 2, 3); // 6
16+ onceFn(2, 3, 6); // undefined, fn was not called
17+ Example 2:
18+
19+ Input: fn = (a,b,c) => (a * b * c), calls = [[5,7,4],[2,3,6],[4,6,8]]
20+ Output: [{"calls":1,"value":140}]
21+ Explanation:
22+ const onceFn = once(fn);
23+ onceFn(5, 7, 4); // 140
24+ onceFn(2, 3, 6); // undefined, fn was not called
25+ onceFn(4, 6, 8); // undefined, fn was not called
26+
27+
28+ Constraints:
29+
30+ calls is a valid JSON array
31+ 1 <= calls.length <= 10
32+ 1 <= calls[i].length <= 100
33+ 2 <= JSON.stringify(calls).length <= 1000
34+
35+
36+
37+
38+
39+
40+ */
41+ 42+ 43+ 44+ type JSONValue = null | boolean | number | string | JSONValue [ ] | { [ key : string ] : JSONValue } ;
45+ type OnceFn = ( ...args : JSONValue [ ] ) => JSONValue | undefined ;
46+ 47+ function once ( fn : Function ) : OnceFn {
48+ let called = false ;
49+ let rst : JSONValue ;
50+ 51+ return function ( ...args : JSONValue [ ] ) : JSONValue | undefined {
52+ if ( ! called ) {
53+ called = true ;
54+ rst = fn ( ...args ) ;
55+ return rst ;
56+ }
57+ return undefined ;
58+ } ;
59+ }
60+ /**
61+ * let fn = (a,b,c) => (a + b + c)
62+ * let onceFn = once(fn)
63+ *
64+ * onceFn(1,2,3); // 6
65+ * onceFn(2,3,6); // returns undefined without calling fn
66+ */
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