perf: avoid repeated evaluation in each function call (#39)

Author: ybiquitous

index.ts

@@ -42,13 +42,14 @@ export default function bem(
     prefix = defaults.prefix,
   } = {},
 ) {
-  return (elementOrModifiers?: string | Modifiers, modifiers?: Modifiers) => {
-    if (namespace && prefix) {
-      throw new TypeError(`prefix('${prefix}') is deprecated. Use namespace('${namespace}') instead.`)
-    }
+  if (namespace && prefix) {
+    throw new TypeError(`prefix('${prefix}') is deprecated. Use namespace('${namespace}') instead.`)
+  }
 
-    const nsDelim = namespace ? namespaceDelimiter : ''
-    const pre = prefix || `${namespace}${nsDelim}`
+  const nsDelim = namespace ? namespaceDelimiter : ''
+  const pre = prefix || `${namespace}${nsDelim}`
+
+  return (elementOrModifiers?: string | Modifiers, modifiers?: Modifiers) => {
     let base = `${pre}${block}`
 
     if (!elementOrModifiers) {

test.ts

@@ -144,9 +144,9 @@ testCases.forEach(({ description, tested, expectations }) => {
 
 describe('`namespace` and `prefix` at the same time', () => {
   it('throws `TypeError`', () => {
-    const b = block('block', { namespace: 'ns', prefix: 'pre' })
-    expect(() => b()).toThrow(TypeError)
-    expect(() => b()).toThrow("prefix('pre') is deprecated. Use namespace('ns') instead.")
+    const fn = () => block('block', { namespace: 'ns', prefix: 'pre' })
+    expect(fn).toThrow(TypeError)
+    expect(fn).toThrow("prefix('pre') is deprecated. Use namespace('ns') instead.")
   })
 })