Add prepend util

docs/lib/tools.md

@@ -218,6 +218,9 @@ export declare const take: <T>(n: number, xs: Iterable<T>) => T[];
 // Concatenates multiple arrays, filtering out null/undefined
 export declare const concat: <T>(...xs: T[][]) => T[];
 
+// Prepends element to array
+export declare const prepend: <T>(x: T, xs: T[]) => T[];
+
 // Appends element to array
 export declare const append: <T>(x: T, xs: T[]) => T[];
 

packages/lib/src/Tools.ts

@@ -430,6 +430,14 @@ export const concat = <T>(...xs: T[][]) => xs.flatMap(x => (x === undefined ? []
  */
 export const append = <T>(x: T, xs: Iterable<T>) => concat(Array.from(xs), [x])
 
+/**
+ * Prepends element to array
+ * @param x - Element to prepend
+ * @param xs - Array to prepend to
+ * @returns New array with element prepended
+ */
+export const prepend = <T>(x: T, xs: Iterable<T>) => concat([x], Array.from(xs))
+
 /**
  * Creates union of two arrays
  * @param a - First array