1. A computer system for an isolated program execution environment, the computer system including a server having a processor, the processor configured to: receive, by the isolated program execution environment on a computer comprising a processor and a memory, a request to run a program, wherein the program comprises a JavaScript program, and wherein the program is executed in a strict mode; wrap program code corresponding to the program as a function; clone a real global object of the isolated program execution environment to create a fake global object; pass the fake global object to the function; and execute the function, such that the function executes the program, wherein the program accesses data or a variable in the fake global object during execution, and does not access the real global object; discard the fake global object after execution of the program is completed; determine that the program requires an external module; determine whether the external module is stored in an immutable module cache of the isolated program execution environment; based on determining that the external module is stored in the immutable module cache, pass a reference to a cache entry corresponding to the external module to the program; execute the external module by the program, wherein the program does not modify the external module during execution; wherein, based on determining that the external module is not stored in the immutable module cache: retrieve program code corresponding to the external module from a memory; compile the retrieved program code to create a compiled module; seal the compiled module so that the sealed compiled module is immutable; store the sealed compiled module in the immutable module cache; execute a plurality of programs simultaneously by the isolated program execution environment, wherein each of the plurality of programs has a respective fake global object.